Monday, February 17, 2020

Sex declines, but engagement in, or being target of, dominant behaviors (spanking, choking, name calling, performing aggressive fellatio, facial ejaculation, penetration without first asking/discussing) seems more frequent

Sex frequency and satisfaction seems to be declining in the West:
Declining Sexual Activity and Desire in Women: Findings from Representative German Surveys 2005 and 2016. Juliane Burghardt et al. Archives of Sexual Behavior, December 4 2019.
How strong the tendency among Finns still is to form only one, life-long relationship? Changes in how many partners they have, same-sex experiences, masturbation, etc. Monogamy vs Polygamy. Osmo Kontula. SexuS Journal, Winter-2019, Volume 04, Issue 11, Pages 959-978.

, but roughness seems to be increasing:

Herbenick D, Fu T-C, Wright P, et al. Diverse Sexual Behaviors and Pornography Use: Findings From a Nationally Representative Probability Survey of Americans Aged 14 to 60 Years. J Sex Med 2020;XX:XXX–XXX, Feb17 2020.

Background  Convenience sample data indicate that substantial portions of adults have engaged in sexual behaviors sometimes described as rough; little is known about these behaviors at the population level.

Aim  To describe, in a U.S. probability sample of Americans aged 18 to 60 years, (i) the prevalence of diverse sexual behaviors, described here as dominant and target behaviors; (ii) the age at first pornography exposure as well as prevalence, range, and frequency of pornography use; (iii) the association between past year pornography use frequency and dominant/target sexual behaviors; and (iv) associations between lifetime range of pornography use and dominant/target sexual behaviors.

Methods  A confidential cross-sectional online survey was used in this study.

Outcomes  Lifetime engagement in dominant behaviors (eg, spanking, choking, name calling, performing aggressive fellatio, facial ejaculation, penile-anal penetration without first asking/discussing) and lifetime engagement in target behaviors (eg, being spanked, being choked, being called names during sex, having their face ejaculated on, receiving aggressive fellatio, or receiving penile-anal penetration without having discussed) were assessed; lifetime pornography use, age at first porn exposure, past-year frequency of porn viewing, and lifetime range of pornography were also assessed.

Results  Women as well as men who have sex with men were more likely to report target sexual behaviors: having been choked (21.4% women), having one's face ejaculated on (32.3% women, 52.7% men who have sex with men), and aggressive fellatio (34.0% women). Lifetime pornography use was reported by most respondents. After adjusting for age, age at first porn exposure, and current relationship status, the associations between pornography use and sexual behaviors was statistically significant.

Clinical Implications  Clinicians need to be aware of recent potential shifts in sexual behaviors, particularly those such as choking that may lead to harm.

Strengths & Limitations  Strengths include U.S. probability sampling to provide population level estimates and the use of Internet-based data collection on sensitive topics. We were limited by a lack of detail and context related to understanding the diverse sexual behaviors assessed.

Conclusion  Clinicians, educators, and researchers have unique and important roles to play in continued understanding of these sexual behaviors in the contemporary United States.

Check also previous work by some of these authors:

Feeling Scared During Sex: Findings From a U.S. Probability Sample of Women and Men Ages 14 to 60. Debby Herbenick, Elizabeth Bartelt, Tsung-Chieh (Jane) Fu, Bryant Paul, Ronna Gradus, Jill Bauer et al. Journal of Sex & Marital Therapy , Volume 45, 2019 - Issue 5, Pages 424-439, Apr 4 2019.

Abstract: Using data from a U.S. probability survey of individuals aged 14 to 60, we aimed (1) to assess the proportion of respondents who ever reported scary sexual situations and (2) to examine descriptions of sexual experiences reported as scary. Data were cross-sectional and collected via the GfK KnowledgePanel®. Scary sexual situations were reported by 23.9% of adult women, 10.3% of adult men, 12.5% of adolescent women, and 3.8% of adolescent men who had ever engaged in oral, vaginal, or anal sex. Themes included sexual assault/rape, incest, being held down, anal sex, choking, threats, multiple people, novelty/learning, among others.


Our study provides insights into sexual experiences that 14- to 60-year-old Americans have found scary. Rather than investigate a specific behavioral category (e.g., sexual assault or coercion), we chose to center on respondents’ emotional lives and ask about scary experiences. We hoped to understand more about sexual events that may be problematic, hurtful, or frightening and that may otherwise be missed in research focused only on sexual violence. Our research questions were informed by having heard from students and interview participants, over a number of years, about frightening sexual experiences that seemed to go beyond “bad hook-ups” (Littleton, Tabernik, Canales, & Backstrom, 2009). Sometimes people would describe sexual encounters that were frightening in their entirety; other times, the frightening experience occurred within an otherwise pleasurable sexual encounter. We aimed to augment the literature by addressing both the prevalence of scary sexual experiences and respondents’ descriptions of these.

We were struck, but not surprised, at the gendered aspects of our findings. Substantially more women than men reported that someone had done something during sex that had scared them. This is likely because scary things truly do happen more often to women than men during sex. This is important considering that women’s allegations of assault are still sometimes characterized as simple “misunderstandings” (e.g., Kitchener, 2018; Levin, 2016). We note that some of the men’s descriptions of scary sex (e.g., that referred to menstruation, adolescent/learning curve, first coitus, wondering whether the person who’s performing oral sex is friends with a prior partner) differed considerably from examples more often provided by women that pertained to rape, forced sex, being held down, threatened with weapons, choked, and painful sex that one asks to stop but that does not stop. Even among the pregnancy/STI risk responses, women’s experiences more often alluded to feeling pressured into unprotected sex. In contrast, men’s responses in this category described forgetting to use a condom, not knowing about a partner’s sexual history, or finding out a female partner had many prior sex partners. And yet, the overall proportion of men who reported ever experiencing sex as scary was nearly half that of women; subsequent research might focus on better understanding the range of (perhaps underreported) male trauma.

We identify these gendered differences not to dismiss the experiences of men who had sex with women and who felt scared about some aspect of sex, but to highlight the different kinds of sex (and perhaps the different kinds of fear) that people grapple with during their sexual exploration and development. Those who have not generally experienced alarming aspects of sex, moments in which they wondered whether they were about to be raped or killed, may find it difficult to imagine how scary sex can feel for others. Consequently, some may find it difficult to empathize with what it means to be a sexually active woman in a society where quite scary things happen; we are reminded of Gavey (2012), who wrote about “sexuality in a sexist world.”

We note, too, that quite a few of the men who described scary sexual situations (especially those that involved physical aggression or forced penetration) alluded to the other person being male. This finding is consistent with literature demonstrating that gay and bisexual men are at disproportionately greater risk of sexual violence (Rothman, Exner, & Baughman, 2011). That adults who report lower household income were more likely to report having had scary sexual experiences should be further interrogated. It may be that the scary experiences were traumatic in ways that altered life and career paths for some individuals. Others may have experienced scary sexual experiences as part of a larger pattern of disadvantage or differential opportunity or may have put up with sexual coercion because they felt they had limited options.

We were struck by how often the scary experiences reflected interpersonal violations of the sexual rights identified by WAS (World Association for Sexual Health, 2014). For example, those who wrote about their partner continuing sex after being asked to stop experienced a violation of their autonomy. Respondents who wrote about forced sex, sexual assault, or rape experienced a violation of their right to be free from sexual violence and coercion. In addition, those whose request to use contraception was ignored experienced violations of rights related to making their own reproductive choices. Because the scary experiences were also disproportionately reported by women as well as by men who have sex with men, collectively they reflect sexual inequities derived from societal inequities. Sex that disproportionately frightens women and other minoritized individuals reflects cultural and social issues that need to be reckoned with.

Our findings add to a body of literature that, as described earlier, demonstrates a privileging of men’s sexual pleasure, with women more often reporting lower levels of sexual pleasure and arousal, less frequent orgasm (Herbenick et al., 2010b), more frequent pain (Herbenick et al., 2015) and, described here, more common experiences of frightening sex. Indeed, scholars have long described how women’s economic and political insubordination impacts their sexual lives and opportunities (e.g., Tiefer, 2001), making them vulnerable to sex that feels scary, painful, or beyond one’s ability to control or consent to (e.g., Bay-Cheng, 2010; Gavey, 2012; Tiefer, 2001).

Even in consensual sex, women more often give reasons for having sex that include feeling pressured, obligated, or at the insistence of one’s partner (Meston & Buss, 2007). In addition, many young women choose to continue having vaginal intercourse even when it is painful, not wanting to spoil sex for their partner by interrupting sex (Elmerstig, Wijma, & Swahnberg, 2013). What we know less of is how this comes to be. How common is it for women to begin their sexual lives believing that sex should continue at all costs, even if it hurts? Alternatively, is this something that some women learn from trying to stop painful intercourse only to have their partner ignore their request?

Aside from sexual assault and rape, some of the most common descriptions of sexual situations that respondents found scary involved anal sex or choking. Indeed, in the past few decades, the prevalence of Americans reporting lifetime anal sex has nearly doubled, even as the frequency has remained low (Herbenick et al., 2010a). For such a common behavior, we found it striking how commonly anal sex was included in descriptions of scary sex. Subsequent research might further explore this. To what extent is anal sex scary at first but then evolves into a neutral or even pleasurable experience? What do individuals find scary about anal sex? For some respondents, it seems that the scary aspect was that they had already communicated to their partner that they did not want to have anal sex, but their partner coerced or forced anal sex anyway. For others, their partner did not stop after they were asked to stop or else anal sex got too rough, suggesting that anal sex itself might not necessarily have been scary had it been wanted, consensual, and better communicated about. (By “communicated” we acknowledge the critical roles of both verbal communication and listening/responding.) Indeed, in a series of studies of college men and women, it was not uncommon for women to indicate that their refusals to engage in various sexual behaviors were often met with men expressing displeasure, anger, or continuing with sexual advances despite the woman’s expressed wishes to stop (see Byers, 1996, for overview).

Choking and other aggressive behaviors (such as hitting and forceful hair pulling) were also often described among the scary sexual experiences. Like anal sex, choking appears to have become more commonly portrayed in sexually explicit media and sexual choking behaviors (and interest in choking) are associated with pornography use (Bridges, Sun, Ezzell, & Johnson, 2016; Sun, Wright, & Steffen, 2017). In recent years, choking and various forms of breath restriction/breath play have also become a part of nonsexual games that some adolescents engage in (Linkletter, Gordon, & Dooley, 2010). However, choking and breath play are associated with serious risks—including accidental death—and thus it is not surprising to see choking often described as scary. Most of the choking instances described appear to have not been discussed by partners in advance; the other person just started choking the respondent. Consequently, some worried they were being strangled: a common form of intimate partner violence, especially committed against women who partner with men (Messing, Thomas, Ward-Lasher, & Brewer, 2018). Given how little is known about the role of choking in contemporary sexual repertoires, we encourage subsequent research to consider questions such as the following: How do people first learn about choking during sex? To what extent are sexually explicit media contributing to sexual behaviors such as choking? What do they learn about it? What kinds of conversations do sexual partners have about choking?

Our data provide opportunities for sexuality therapists, educators, parents, and couples to think about the many different ways that sexuality can be experienced. Findings underscore that unwanted, unpleasurable, and even frightening things can happen during sex that is otherwise wanted and pleasurable. Even well-meaning partners can err by introducing sex toys or sex acts that the other person does not like. Our data also speak to the importance of listening to one’s sex partner, that is, not doing something that they have said they do not want to do, stopping sex when they ask to stop, and using condoms or other contraceptives if they ask to do so. Clinicians have a critical role to play in supporting clients to create consensual and mutually pleasurable sexual lives. Clinicians might also consider asking clients about their past experiences with scary or frightening sexual experiences, as sexual history guides often address assault or rape but few other kinds of negative situations.

Strengths and limitations

A strength of our study is that we used the GfK KnowledgePanel, which uses probability-based sampling for panel establishment and is intended to be nationally representative of the English-speaking, non-institutionalized U.S. population. This method was well suited to establishing the prevalence of having experienced scary sexual situations among the U.S. population. We took care to communicate to parents the aims of our study and how we would go about showing questions to their adolescent sons and daughters that were appropriate for their sexual development and experiences to date. Yet, it is possible that parents chose not to allow us to survey their child if they felt their son or daughter was sexually naïve or if they knew they had experienced sexual trauma. As with any study, we can imagine varied reasons that some people may choose not to participate in a study or to allow their child to do so. Given the small number of adolescent reports, these estimates should be viewed with caution and subsequent research should examine adolescents’ scary sexual experiences in greater detail. We also have no way of knowing the full range of scary experiences that respondents had actually endured, and some respondents may have chosen to share a relatively neutral example rather than share something particularly frightening to them. Conversely, some respondents may have chosen to share the worst or more frightening example.

We also collected data over the Internet, which has been shown to facilitate reporting on sensitive topics (Burkill et al., 2016). Yet, a limitation of Internet-based surveys is that they tend to yield less detail than do in-person interviews (some people declined to provide examples of scary situations and others wrote only brief responses). Subsequent research might utilize interviews to better understand the details of individuals’ scary sexual experiences. For this initial study, we were content to leave the idea of feeling “scared” open to interpretation so that we could understand a range of experiences. However, subsequent research may want to differentiate feeling scared about one’s safety versus scared about novel experiences.

Due to budget-related survey space restrictions, we only asked two questions about scary sexual experiences. We did not ask about respondents’ age at the time of the experience, the gender(s) of those involved, or other details. It is unclear how recall bias may have factored into respondents’ endorsement of having experienced scary sexual situations. We hope that our study, in presenting sexual situations experienced as scary, may inform the development of more nuanced research going forward. Subsequent research might investigate how scary situations are resolved (if/when they are) and how they become incorporated into individuals’ sexual learning (e.g., what changes, if any, people make in their lives following scary experiences). Researchers might also examine how people interpret scary sexual situations (including how they may be recast over time), including the extent to which they feel guilt, shame, or anger or alternatively minimize their experience, as commonly occurs in sexually coercive situations (Jeffrey & Barata, 2017).

Our study is not able to address the prevalence of particular types of scary sex. For example, we cannot speculate what percentage of Americans have experienced choking and felt scared by it. Respondents could provide any example of a scary sexual experience, which may have been one of their more frightening or one markedly less so. Our data might inform the development of future checklists of potentially scary situations. The use of such a checklist would be better able to address population-based prevalence of specific experiences.

Children's understanding of dominance and prestige in China and the UK

Children's understanding of dominance and prestige in China and the UK. Anni Kajanus, Narges Afshordi, Felix Warneken. Evolution and Human Behavior, Volume 41, Issue 1, January 2020, Pages 23-34.

Abstract: Individuals can gain high social rank through dominance (based on coercion and fear) and prestige (based on merit and admiration). We conducted a cross-cultural developmental study and tested 5- to 12-year-olds, and adults in the UK and China, aiming to determine (a) the age at which children distinguish dominance and prestige, and (b) the influence of cultural values on rank-related reasoning. We specifically tested participants in China because of the value of prestigious individuals modestly yielding to subordinates, a social skill that becomes more salient with age. In both populations, the distinction between dominance and prestige emerged at five years, and improved over childhood. When reasoning about a resource conflict between a high-ranking party and a subordinate, adults in both countries expected high-rank individuals to win, although Chinese adults were less likely to do so regarding prestigious individuals. Across the two countries, younger children (5–7 years) responded similarly to each other, not favoring either party as the winner. Older children (9–12 years), however, diverged. Those in the UK chose the high-rank party, while those in China made no systematic inference. Overall, our findings suggest that while children distinguish prestige and dominance comparably in the two countries, they develop culturally-influenced expectations about the behavior of high-rank individuals.

5. General discussion

Our aims were to test: (1) the age at which children differentiate dominance and prestige; and (2) the influence of cultural ideas of hierarchy and conflict on expectations about the behaviors of dominant and prestigious individuals. The experiments yielded several key findings for both lines of inquiry.
(1)  Developmental trajectory
First, 5- to 12-year-olds in both the UK and China easily identified dominant and prestigious characters as high-ranking. Thus, even for the younger group of children (5–7 years), cues to prestige (e.g. asking for advice, following said advice) were enough to merit inferences about rank. In fact, children were just as successful at recognizing that the prestigious character was higher-ranking as they were for the dominant character. Next, even younger children distinguished between prestigious and dominant characters in a third-party situation, choosing the prestigious character significantly more when asked whom the subordinate would approach or like, than when asked whom she feared. This finding is particularly interesting in light of the fact that children do not shun coercive (or dominance-like) strategies in their peer groups until eight years (Hawley, 1999). Since our younger group was younger than eight years, the finding suggests that shunning coercive strategies is not the result of noticing the difference between dominant and prestigious strategies. Nonetheless, the ability to distinguish dominance and prestige improved with age. Finally, there were no differences between responses in the UK and China, providing some of the first empirical evidence of children from non-Euro-American cultures understanding cues to rank similarly to children in Europe and the US. In summary, children attributed a combination of traits to the characters that are reflective of a conceptual distinction between dominance and prestige, viewing both as having high social rank, but differing in prestigious characters being liked and approached versus dominant individuals being feared and avoided.
One alternative explanation of the findings from Experiment 1 is that children did not actually distinguish dominance from prestige, and instead succeeded in the task by answering the questions (age, liking, approach, fear) piecemeal. In other words, maybe they answered that Dimo would like and approach the prestigious character simply because the character seemed nice and friendly, and feared and avoided the dominant character because that one seemed mean and aggressive. Similarly, children could have inferred age (i.e. rank) by drawing on cues like being imitated. We do not dispute that these cues led children to answer the questions correctly. In fact, we claim that this is exactly what the understanding of prestige and dominance looks like: an understanding of this combination of features. None of these individual features differentiates between dominance and prestige, but one person having high status while being nice and approachable differentiates this person from a similarly high-ranking individual who is mean and aggressive.
Although younger children recognized the rank difference between the characters just as easily as older children (Experiment 1), they did not infer that higher-ranking parties would win resource conflicts (Experiment 3). This failure cannot be attributed to a cultural effect, as children in the UK and China performed similarly. When asked to justify their choice of who would win the conflict, almost no children referenced rank in their explanations, confirming that their failure to infer is a real consequence of how they construed the scene. Thus, although younger children extracted rank from watching interactions between characters in Experiment 1, they were unable to automatically incorporate them into inferences about subsequent behaviors in Experiment 3. Future work should explore this finding further.
(2)  Cross-cultural differences
A key contribution from our studies is evidence for the influence of cultural norms and value systems on how children and adults understand social hierarchies and reason about them. Adults in the UK and China were similar in that they both inferred that high-ranking characters would win against a subordinate. They did differ, however, in the degree to which they made this inference in the prestige case. Chinese adults were less likely than British adults to think that the prestigious person would win the resource. This difference, while subtle, is a key sign of the cultural difference reflecting the value specifically placed on yielding to others when in a position of prestige (Kajanus, n.d.). The cultural difference also manifested in older children (9–12 years), but in a different way. Older children in the UK inferred that the high-ranking party would win the conflict, regardless of whether the character was prestigious or dominant. In contrast, older children in China responded similarly to younger children in both countries, demonstrating no systematic prediction about who would win in either of the conflict cases. But unlike younger children's explanations, which were shallow and unrelated to social rank, older children in China and the UK provided similar levels of rank-relevant explanations (around 65%). Consequently, the lack of systematic inferences was age-driven in younger children, but culturally influenced in older children in China. Moreover, older children in China who thought the subordinate would win the conflict were more likely to mention the prestigious character yielding than the dominant character yielding. This finding aligns with the value of yielding by prestigious individuals. Together, these findings suggest that the difference between older children in China and the UK is rooted in a cultural difference.
On the topic of Chinese participants' choices, two points merit further discussion. With regard to adults, the fact that Chinese participants thought that both dominant and prestigious parties would win the conflict gives room for pause, given the value of prestigious individuals showing restraint and giving up resources to lower-status parties. However, it is important to note that yielding to those in lower positions is a sophisticated social skill highly dependent on the nuances of the situation, such as the dynamics of ascribed status hierarchies and the importance of the issue at the root of the conflict (Kajanus, n.d.). Thus, most adults may have viewed the situation as one in which the high-ranking character lacked the sophistication to yield, given the cartoonish appearance of the characters, the simplicity of the exchanges, and the prestigious character's unabashed and juvenile desire for the resource. This may have made the high-ranking character's win the more straightforward choice. A number of them may have, nonetheless, considered the situation in terms of the prestigious character yielding, thereby bringing about the difference with the British adults.
Another puzzling finding has to do with the older Chinese children's responses in Experiment 3. In a reverse situation from the adults, children held no expectation of either party winning in either conflict case. The lack of a clear prediction may be understandable for the prestige case, but is more surprising in the dominance case. As the data do not clarify the reason for this result, we can only speculate. One likely account is that children at this age are not yet fully proficient in the complexities of rank relations and the rules of yielding. They have learned that high-ranking persons will sometimes yield, but the ways in which personality characteristics (e.g. prestige, dominance) and ascribed hierarchies (position, age, gender, etc.) factor into yielding are still unclear. It is therefore possible that even though they distinguish between dominant and prestigious processes, in relation to yielding they simply treat them similarly as signs of high rank. It is notable that even adults of both countries treated the prestige and dominance cases the same in their predictions about yielding. This does not mean that they cannot distinguish between prestige and dominance as bases of high rank. Unlike Experiment 1, which specifically tested this ability, Experiments 2 and 3 were focused on predictions about yielding in a conflict between high-ranking and low-ranking character. Even so, Chinese adults were less likely to choose the prestigious character as the winner than the UK adults. Older children also showed signs of making the distinction in relation to yielding, as they were more likely to mention the higher-ranking character yielding in the prestige scene than in the dominance scene.
One limitation of the present studies was the sample size (n = 40 per age, country), which although on par with previous work on children's reasoning about social rank (e.g. Charafeddine et al., 2015, Charafeddine et al., 2016), was not very large. Although a larger sample would have been more beneficial, the number of children in the schools we had access to logistically limited us. We hope to remedy this in our future work.
Relying on large amounts of ethnographic evidence, Fiske (1992) laid out a theory of four elementary forms of social relationships in human societies, one of which is authority ranking, which corresponds to hierarchical relationships marked by rank differences. Even though the relational model of authority ranking might be universal, it can be implemented in myriad ways. Children may be endowed with an innate knowledge of the model of hierarchical relationships (Thomsen & Carey, 2013), but they must learn its details and exactly how it operates in their particular social milieu. These experiments provide evidence for one such culturally-influenced aspect of hierarchical relationships across two cultural backdrops. The range of cultural features and cues to relationships is underexplored, as is how children come to learn them and how long it takes them to do so. We offer an initial glimpse of one particular aspect and hope that future work will go on to uncover many more.

Forager-horticulturists: Lowering the voice fundamental frequency in audio clips of men speaking increased perceptions of fighting ability but did not affect perceptions of prestige and decreased their attractiveness to women

Sexual selection for low male voice pitch among Amazonian forager-horticulturists. Kevin A. Rosenfield et al. Evolution and Human Behavior, Volume 41, Issue 1, January 2020, Pages 3-11.

Abstract: Pitch is the most perceptually salient feature of the voice, yet it is approximately five standard deviations lower in men than in women, a degree of sexual dimorphism exceeding that of all extant nonhuman apes. Evidence from Western samples suggests that low-frequency vocalizations may have augmented male mating success ancestrally by intimidating competitors and/or attracting mates. However, data are lacking from small-scale societies. We therefore investigated sexual selection on male pitch (measured by fundamental frequency, fo) in a population of Bolivian forager-horticulturists, the Tsimané. We found that experimentally lowering fo in audio clips of men speaking increased perceptions of fighting ability but did not affect perceptions of prestige and decreased their attractiveness to women. Further, men with lower speaking fo reported higher numbers of offspring, and this was mediated by the reproductive rates of men's wives, suggesting that men with lower fo achieved higher reproductive success by having access to more fertile mates. These results thus provide new evidence that men's fo has been shaped by intrasexual competition.

Who teaches children to forage among Hadza and BaYaka Hunter-Gatherers of Tanzania and Congo? Child-to-child teaching was more frequent than adult-child teaching; and children taught more with age

Who teaches children to forage? Exploring the primacy of child-to-child teaching among Hadza and BaYaka Hunter-Gatherers of Tanzania and Congo. Sheina Lew-Levy et al. Evolution and Human Behavior, Volume 41, Issue 1, January 2020, Pages 12-22.

Abstract: Teaching is cross-culturally widespread but few studies have considered children as teachers as well as learners. This is surprising, since forager children spend much of their time playing and foraging in child-only groups, and thus, have access to many potential child teachers. Using the Social Relations Model, we examined the prevalence of child-to-child teaching using focal follow data from 35 Hadza and 38 BaYaka 3- to 18-year-olds. We investigated the effect of age, sex and kinship on the teaching of subsistence skills. We found that child-to-child teaching was more frequent than adult-child teaching. Additionally, children taught more with age, teaching was more likely to occur within same-sex versus opposite-sex dyads, and close kin were more likely to teach than non-kin. The Hadza and BaYaka also showed distinct learning patterns; teaching was more likely to occur between sibling dyads among the Hadza than among the BaYaka, and a multistage learning model where younger children learn from peers, and older children from adults, was evident for the BaYaka, but not for the Hadza. We attribute these differences to subsistence and settlement patterns. These findings highlight the role of children in the intergenerational transmission of subsistence skills.

6. Discussion

The present study aimed to investigate how age, sex, and kinship influenced the teaching of subsistence skills in BaYaka and Hadza forager 3- to 18-year-olds. Our findings suggest that Hadza and BaYaka children participated in teaching, either as a teacher or as a learner, between 6 and 8 times an hour. A majority of these teaching events occurred within child dyads. Alongside research among the Aka and Ngandu (Boyette & Hewlett, 2017a; Hewlett & Roulette, 2016), Baka (Gallois, Duda, Hewlett, & Reyes-garcía, 2015), Maya (Maynard, 2002; Zarger, 2002), and Fijians (Kline, 2016), our results highlight the central role Hadza and BaYaka children play as teachers, and not just acquirers, of cultural knowledge.
Children in both populations taught more with age, with overall teaching directed to children peaking in adulthood. Teaching likely develops with age because children's teaching abilities continue to increase, and because they have more knowledge to share with others (Strauss & Ziv, 2012). Though the development of children's teaching abilities have been documented in multiple societies in the industrialized west (see Strauss & Ziv, 2012 for review), our findings lend support to a growing body of evidence demonstrating that in non-western societies, this development occurs independently of intensive formal schooling (Boyette & Hewlett, 2017a; Maynard & Tovote, 2009). Interestingly, after approximately 30, the teaching of children actually decreased with age. Since, by 30, most adults have children who are old enough to teach their younger siblings, our findings may reflect children's participation in offsetting their cost of care. Children's participation in economic activities among the Maya likely increases mother's reproductive success (Lee & Kramer, 2002). By accelerating other children's subsistence knowledge acquisition through teaching, children may be increasing their inclusive fitness by promoting sibling self-sufficiency and shortening parental inter-birth interval. Children may also be improving their individual fitness by increasing their share of parental provisioning. Furthermore, children may liberate parents to teach more complex skills to adolescents and other adults, thus reducing the cost of cumulative cultural transmission.
Consistent with kin selection theory, teaching was more likely to occur between related dyads than unrelated dyads in both groups. However, when compared to non-kin and other-kin, sibling teaching was more common among the Hadza than among the BaYaka. We interpret these findings as indicating that teaching was more likely to occur within nuclear families among the Hadza compared to the BaYaka. We propose that these findings are related to camp structure. As noted earlier, BaYaka camps are typically more compact than Hadza camps (Hewlett et al., 2019) partially because of the constraints imposed by living in a forested environment rather than in the savannah. As a result, BaYaka children are invariably in closer proximity to all other camp members while in camp, while Hadza children can more easily assort with more closely related individuals, including siblings and parents. This may result in different teaching patterns, where other-kin and non-kin play a greater role in knowledge transmission for the BaYaka, whereas for the Hadza, the nuclear family may play a greater role in knowledge transmission. An alternative explanation may be simply that, because the Hadza have more siblings than the BaYaka, the former experienced more sibling teaching than the latter. However, Blurton Jones (2016) states that the total fertility rate for the Hadza is 5.3, while, for the BaYaka, total fertility rate is reported by Hewlett (1991) as 6.2, with similar infant mortality rates (~20% Blurton Jones, Hawkes, & O'Connell, 2002; Hewlett, 1991). Furthermore, as Table 1 shows, the BaYaka had proportionally more siblings in camp than the Hadza (5% vs. 3%), making it unlikely that number of siblings in camp explains the observed difference in Hadza and BaYaka sibling teaching. Thus, our results suggest that intra-site variation in settlement structure may influence the distribution of kin teaching. Future studies should further investigate this claim.
For the BaYaka only, younger children were more likely to be taught by other children while BaYaka adolescents were more likely to be taught by adults. This finding is consistent with the multistage model of knowledge acquisition, which suggests that children develop basic skills from other children before seeking skilled adults from whom they can update their knowledge, and who might also be more willing to teach individuals with the necessary baseline competence (Henrich & Henrich, 2010; Reyes-García et al., 2016). While our data support a multistage model of learning among the BaYaka, we found little difference in teacher's age for younger and older learners among the Hadza. While unexpected, this finding may be explained by examining foraging participation. Hadza children collect between 25 and 50% and sometimes even 100% of their daily caloric needs from an early age (Crittenden et al., 2013; Hawkes et al., 1995). Although children tend to target easier to access resources such as berries and baobab when they are younger, they are provided with opportunities to practice more complex resource acquisition throughout childhood; for example, boys as young as two are made small, functional bows and arrows, and girls are provided with small, appropriately sized digging sticks (Crittenden, 2016). Unlike among the BaYaka, children are fully expected to collect food with these tools. Thus, for the Hadza, teaching by adults may primarily occur through stimulus enhancement in early life, after which children are more likely to learn complex skills through participation in foraging with other children than through teaching by adults. Though a multistage learning model where children learn with other children when younger, and by adults when older may be more common, it may nonetheless depend on the foraging niche in which learning occurs. Future studies should thus take seriously the role of ecological context when investigating the distribution of learning processes across the lifespan.
Mathematical models investigating optimal learning strategies suggest that individual learning should occur only after children have acquired knowledge socially (Aoki, Wakano, & Lehmann, 2012; Borenstein, Feldman, & Aoki, 2008; Lehmann et al., 2013). Although previous studies of play (Bock & Johnson, 2004), observation (Greenfield, 2004), and teaching (Boyette & Hewlett, 2017a) found that social learning declined with age, presumably because older individuals have begun to refine learned behavior through individual practice, our final model found only a weak negative relationship between learner's age and teaching. However, we note that learner's age was a strong negative predictor in additional models which omitted this interaction (see supplementary materials). This suggests that what might first appear to be a decreasing likelihood for older individuals to be learners is actually better explained by (a) a decrease in teaching by older individuals, due to the declining latter portion of the quadratic ‘teacher age’ curve (Fig. 1), and (b) a tendency for teachers and learners to be of similar ages, as indicated by the positive teacher/learner age interactions. In other words, the decline in teaching by older individuals is sufficient to explain the decline in learning by older individuals as well.
As in other aspects of forager life (Allen-Arave et al., 2008; Crittenden & Zes, 2015; Peterson, 1993), we found evidence for high dyadic reciprocity, and a large effect of the dyad, in teaching. Researchers working with highly stratified cultures have found collaboration to enhance children's knowledge acquisition in experimental settings (Dean, Kendal, Schapiro, Thierry, & Laland, 2012; Dunn, 1983; Laland, 2004; Tomasello, 1999; Tomasello et al., 1993; Wood, Wood, Ainsworth, & Malley, 1995). When comparing collaborative problem solving across cultures, Nielsen, Mushin, Tomaselli, and Whiten (2016) found that Australian Indigenous children collaborated significantly more than Brisbane pre-schoolers (see also Rogoff, 1998). Since collaborative learning generates new knowledge forms (Tomasello, 1999; Tomasello et al., 1993), it may be especially adaptive to foragers relying on unpredictable resources. One limitation of our study is that we examined short-term reciprocity. A long term examination of teaching may show a different, and more unidirectional, pattern. Nonetheless, future studies should examine the advantages conferred by reciprocal knowledge sharing during daily interactions in childhood.
Finally, same-sex teaching was hypothesized to increase the likelihood that children would learn sex-specific skills (Henrich & Gil-White, 2001). Same-sex bias in learning has been noted among foragers the world over (Boyette & Hewlett, 2017a; Draper, 1975; Hewlett & Cavalli-Sforza, 1986; Lew-Levy, Lavi, Reckin, Cristóbal-Azkarate, & Ellis-Davies, 2018; MacDonald, 2007b). Here, we also found strong evidence for same-sex teaching among both the BaYaka and the Hadza.

7. Implications

Taken together, this paper sheds light on the evolutionary importance of, and cross-cultural variation in, child-to-child teaching. Most studies investigating the evolution of childhood have assumed that children require provisioning until at least adolescence (Kaplan et al., 2000), yet recent studies have challenged this claim, showing that children can be, and often are, producers (Bird & Bliege Bird, 2005; Crittenden et al., 2013; Tucker & Young, 2005), that children sometimes produce a surplus of calories which can be shared with the parental generation (Crittenden et al., 2013), and that children's production contributes to parental reproduction (Kramer, 2014; Lee & Kramer, 2002). Similarly, many studies on the evolution of cumulative culture assume that transmission only or primarily occurs from parents to offspring (e.g. Shennan & Steele, 1999), and that childhood is a sensitive period for knowledge acquisition (Kaplan et al., 2000). The results of the present paper problematize these claims because they demonstrate that children are active teachers from an early age. Child-to-child teaching may be especially adaptive because it has the potential to increase children's inclusive and individual fitness by offsetting their own, and their siblings' cost of care (Konner, 1976; Lee & Kramer, 2002). Furthermore, because children can facilitate each other's knowledge acquisition in the zone of proximal development, child-to-child teaching may contribute to more rapid, and potentially less costly, knowledge transfers for basic skills (Hewlett & Cavalli-Sforza, 1986).
Our analysis was limited by the fact that dyadic proximity data proved too difficult to collect while also keeping track of children's teaching and foraging activities. Dyadic proximity is important because individuals may choose to assort with the intent to share knowledge with each other. At least among the BaYaka, adults report inviting children to forage alongside them with the specific intent to teach subsistence skills (Lew-Levy et al., 2019). Similarly, BaYaka children sometimes preferred to forage in the absence of adults so that they could learn with their peers (Boyette & Lew-Levy, Under review). Alternatively, teaching may occur opportunistically while individuals are participating in other cooperative behaviors. For example, parents who forage with their children because they require assistants may also use a foraging trip as an opportunity to teach. Thus, future studies should examine whether teaching is independent from, or a by-product of, other social and cooperative relationships. Future studies should also examine whether cross-cultural differences in associative patterns translates to differences in teaching.
The present paper brings to light several areas for future research. Since fieldwork was only conducted during part of the year, we were unable to observe every foraging activity (e.g. kombi fishing for the BaYaka, weaver-bird collecting for the Hadza); future studies will examine how seasonal variation in child and adult foraging and diet influences how and from whom children learn (Crittenden & Schnorr, 2017; Gallois et al., 2015). In addition, as demonstrated in Table S3, we observed little teaching in especially complex domains, such as hunting and trapping. This may be because these skills are acquired later in life (Gurven, Kaplan, & Gutierrez, 2006; Ohtsuka, 1989; Walker et al., 2002). Since the age cut-off for the present study was approximately eighteen, more longitudinal studies on the distribution of knowledge acquisition across seasons, and in late adolescence and adulthood are needed. Studies comparing teaching to other social learning forms, such as observation and imitation, are also needed. Next, while the present paper considered teaching generally, future studies will examine whether different teaching types covary with the specific domain of subsistence being transmitted. Finally, the foragers with whom we worked had limited exposure to schooling. Future studies will examine how teaching patterns change with increased exposure to schools.

Evolutionary theories suggest that humans prefer sexual dimorphism in faces because masculinity in men & femininity in women may be an indicator of immune function during development; Australian data seems to confirm this

Immune function during early adolescence positively predicts adult facial sexual dimorphism in both men and women. Yong Zhi Foo et al. Evolution and Human Behavior, February 17 2020.

Abstract: Evolutionary theories suggest that humans prefer sexual dimorphism in faces because masculinity in men and femininity in women may be an indicator of immune function during development. In particular, the immunocompetence handicap hypothesis proposes that sexual dimorphism indicates good immune function during development because the sex hormones, particularly testosterone in men, required for the development of sexually dimorphic facial features also taxes the immune system. Therefore, only healthy males can afford the high level of testosterone for the development of sexually dimorphic traits without compromising their survival. Researchers have suggested that a similar mechanism via the effects of oestrogen might also explain male preferences for female femininity. Despite the prominence of the immunocompetence handicap hypothesis, no studies have tested whether immune function during development predicts adult facial sexual dimorphism. Here, using data from a longitudinal public health dataset, the Western Australian Pregnancy Cohort (Raine) Study (Generation 2), we show that some aspects of immune function during early adolescence (14 years) positively predict sexually dimorphic 3D face shape in both men and women. Our results support a fundamental assumption that facial sexual dimorphism is an indicator of immune function during the development of facial sexual dimorphism.