Wednesday, August 5, 2020

Current anthropological hypotheses do not sufficiently explain why habitual concealment of mating evolved in humans but is only seldom exhibited by other social species

Why do human and non-human species conceal mating? The cooperation maintenance hypothesis. Yitzchak Ben Mocha. Proceedings of the Royal Society B: Biological Sciences, Volume 287, Issue 1932, August 5 2020.

Abstract: Despite considerable cultural differences, a striking uniformity is argued to exist in human preferences for concealing sexual intercourse from the sensory perception of conspecifics. However, no systematic accounts support this claim, with only limited attempts to understand the selective pressures acting on the evolution of this preference. Here, I combine cross-cultural and cross-species comparative approaches to investigate these topics. First, an analysis of more than 4572 ethnographies from 249 cultures presents systematic evidence that the preference to conceal mating is widespread across cultures. Second, I argue that current anthropological hypotheses do not sufficiently explain why habitual concealment of mating evolved in humans but is only seldom exhibited by other social species. Third, I introduce the cooperation maintenance hypothesis, which postulates that humans, and a specific category of non-human species, conceal matings to prevent sexual arousal in witnesses (proximate explanation). This allows them to simultaneously maintain mating control over their partner(s) and cooperation with group members who are prevented from mating (ultimate explanations). I conclude by presenting a comparative framework and predictions to be tested across species and human cultures.

3. Hypotheses on the function of concealed mating in humans and their limitations

Widespread human behaviours are strong candidates for evolutionary adaptations. Their functions should therefore also be studied from an evolutionary perspective [5,19]. I thus now turn from an ethnographic survey on the prevalence of concealed mating to an evolutionarily based investigation of its function.

To date, only brief explanations have been suggested for the function of concealed mating (e.g. a few sentences or a footnote). To the best of my knowledge, the first explanation was proposed in 1930 by Malinowski, who argued that public mating ‘excites jealousy. Hence to make love or to eat in public is to invite rivals to seize that which is being enjoyed' ([6], p. 179). Half a century later, Symons repeated a similar argument: ‘Ultimately, this [concealed mating] probably is the outcome of reproductive competition. Where food is scarce, and the sight of people eating produces envy in the unfed, eating is often conducted in private. While there are many societies in which everyone has enough to eat, there are no societies in which everyone can copulate with all the partners he or she desires […] The seeking of privacy for sex probably has been uniformly adaptive and hence is virtually universal among humans' ([2], p. 67).

Building on this interaction between jealousy and reproductive competition, Friedl posited the costly consequence of reproductive competition: ‘the value of hidden sex [is] to protect [the copulating pair] and the social group from the dangers of jealousy caused by competition […] for mates, [as] a degree of social harmony is a prerequisite for an individual animal's reproductive success' ([3], p. 838). Similarly, van Schaik recently hypothesized that ‘the benefit for the man is that it prevents overt contest competition for access to potentially fertile mates, which would threaten male–male cooperation' ([11], p. 184).

These explanations share the view that avoiding overt reproductive competition is the main function of concealed mating in humans, while differing in the importance ascribed to cooperation. Malinowski and Symons neglect the importance of cooperation altogether, Friedl invokes the cost of reproductive competition on ‘social harmony', and van Schaik points out the importance of male–male cooperation. However, if these hypotheses were true, I would expect to find habitual concealment of matings in many other social species. Specifically, I would expect to find concealed matings in our phylogenetically closest living relatives, the social non-human great apes (bonobos Pan paniscus, chimpanzees Pan troglodytes and mountain gorillas Gorilla beringei beringei). In these species, within-group reproductive competition is common (electronic supplementary material, appendix S7), while social cohesion is crucial for between-group competition [20–22] and, at least among chimpanzees, male–male cooperation is vital [22]. Nevertheless, dominant individuals from these species seldom conceal matings from the view of conspecifics (electronic supplementary material, appendix S7; see §4d(iv) and §5 for explanations for occasional concealment in these species).

In conclusion, although the selective pressures proposed by current explanations may have played a role in the evolution of concealed mating, in my opinion, these explanations do not sufficiently explain why habitual concealment evolved in humans but is rarely exhibited in other social great apes. I propose that explaining the currently known taxonomic distribution of concealed mating should be the first touchstone for any hypothesis aiming to explain the function of concealed mating in humans.

4. The cooperation maintenance hypothesis

In this section, I discuss the cooperation maintenance hypothesis (hereafter ‘CMH’). I first review the phenomenon of concealed mating in non-human species, while emphasizing the Arabian babbler (Turdoides squamiceps), for which this hypothesis was originally proposed [23]. Second, I develop a cross-species and cross-cultural account of the hypothesis. Third, I apply the CMH to explain the human case. Fourth, I present predictions to test the CMH across species and human cultures.

(a) Non-human species
In a wide range of species, subordinate animals conceal matings to avoid interference by more dominant group members. Dominant individuals, by contrast, often mate in full view of group members and seldom actively conceal matings (see electronic supplementary material, appendix S7 for review of 34 species and §4d(iv) and §5 for potential explanations). To my knowledge, habitual concealment of matings by individuals that are not subject to physical interruption by conspecifics has so far been documented in two species only: humans and Arabian babblers.

Arabian babblers are cooperative breeding birds living in the Middle East [24]. They live in stable, territorial groups (2–22 individuals) that include all combinations of age, sex and kin relations [24]. During days in which the alpha female ovulates, she is mate-guarded by the alpha male, who may use dominance displays when other males interact with her [25]. Consequently, the dominant pair produces 95% of the offspring [26]. Nonetheless, all group members participate in rearing offspring (e.g. transferring them between shelters [27]) and other cooperative interactions [28]. Helpers have therefore a significant effect on offspring survival [29].

Arabian babblers use discreet communication to initiate copulation [25]. Then, pairs sneak away and only mate in locations and/or times in which they are hidden from view of other group members [23]. Subordinate Arabian babblers are likely to conceal their matings since they risk attack if discovered by more dominant group members [23,25]. A subordinate bird, however, would not attack the dominant pair if it happened to interrupt the privacy of mating [23,25]. After presenting evidence against common explanations for inconspicuous matings in non-human species (see §5), Ben Mocha et al. [23] proposed that dominant Arabian babblers conceal visual stimuli of mating to (i) avoid triggering extra-pair matings by helpers and (ii) maintain alloparental care (by avoiding conflicts that would increase the probability of helpers challenging the alpha position or dispersing).

(b) The cooperation maintenance hypothesis: cross-species and cross-cultural account
The CMH is based on the following argument:


Sensory stimuli of mating between conspecifics evokes sexual arousal and trigger mating behaviour in witnesses (hereafter, the sexual arousal premise).

X (a male and/or a female) tries to control mating access to his/her partner(s) (hereafter, the mating control premise).

X depends on cooperation with group members that he/she prevents from mating with his/her partner(s) (hereafter, the cooperation dependency premise).


Public mating between X and his/her partner will evoke sexual arousal in group members (males and/or females). This, in turn, will increase the likelihood that aroused witnesses will attempt to initiate mating with X's partner when possible (the sexual arousal premise). These attempts will violate X's efforts to control mating access to his/her partner (the mating control premise) and will trigger social conflicts that will harm the cooperation between X and his/her group members (the cooperation dependency premise; figure 1).

[Figure 1. The predicted consequences of concealed and public mating in social systems where an individual tries to control mating access to all or specific partner(s), while also dependent on cooperation with group members that are prevented from mating with his/her partner(s). (Online version in colour.)]

By contrast, sensory concealment of X's mating with his/her partner will not evoke sexual arousal in group members. Hence, the act of mating will not induce extra-pair mating with X's partner and will not affect X's cooperation with other group members (figure 1). I therefore suggest that concealed mating by individuals whose mating is not subject to physical interruption by conspecifics is a relatively non-costly strategy for avoiding unnecessary sexual arousal in group members (proximate explanation). At the ultimate level, concealed mating allows an individual to maintain two needs that would otherwise conflict: mating control over his/her partner(s) and cooperation with those group members that are prevented from mating with these partner(s).

Thus, the CMH elaborates factors that were previously proposed to select for concealed mating—jealousy [2,6], reproductive competition [2,3,11] and social harmony/male–male cooperation [3,11]—and combines them as necessary premises of a coherent argument. According to the CMH, explanations that rely solely on avoiding reproductive competition [2,6] are not sufficient, since the question of concealed mating is only applied to individuals who do not expect interference from conspecifics (e.g. dominant individuals). In addition, in social systems without cooperation, dominant animals settle conflicts with aggression and often mate in public (e.g. Rocky Mountain bighorn sheep Ovis Canadensis [30]). But where competitors also cooperate, aggression may eliminate future cooperation (for ethnographic examples in Yanomamö see [31]). The CMH differs from previous explanations by requiring both a specific form of reproductive competition (i.e. attempting to prevent group members from mating with one's partner) and reliance on cooperation between group members. It thereby highlights the need to manoeuvre between these conflicting motives as the crucial selective pressure.

The CMH further stands up to evolutionary critiques that previous explanations failed to address. Namely, it can explain why dominant individuals of non-human great apes seldom conceal mating: because they rarely monopolize a specific partner (bonobos and chimpanzees use other forms of reproductive competition, e.g. sperm competition [32]; electronic supplementary material, appendix S7]) or they do not depend on cooperation with subordinate group members (mountain gorillas; electronic supplementary material, appendix S7)—at least not to the same extent as humans and Arabian babblers (but see §4d(iv)).

(c) Explaining the human case
In the following, I discuss the evidence that supports the three premises of the CMH in humans.

(i) Sensory stimuli of mating between conspecifics evoke sexual arousal and trigger mating behaviour in witnesses
Visual [33] and auditory [34] stimuli of mating activate the reward system in the human brain and trigger mating behaviour in males and females via mirror neurons. For ethnographic examples, see the Goajiro [35] and Lesu [16].

Knowing that a desired group member has a legitimate mating tie with another person may also trigger jealousy (see the Muria for ethnographic examples [18]). Yet, the sensory stimulus of mating is another powerful trigger of sexual arousal that can be prevented by sensory concealment. The benefits of sensory concealment therefore do not rely on individuals being ignorant of the existence of mating ties between group members.

(ii) X (a male and/or a female) tries to control mating access to his/her partner(s)
Various scholars have claimed that in virtually all cultures, husbands and/or wives try to control mating access to their spouse(s)—at least to a certain degree [1,2,9,36]. Three clarifications should be made regarding this claim. First, mating control should not be confused with monogamy. For instance, a man/woman may marry several wives/husbands and forbid them to have extramarital sex. Second, it has been argued that even in cultures where some extramarital sex is allowed (e.g. in cultures with ‘shared paternity'), husbands and/or wives are still entitled to restrict the trysts of their spouse(s) to specific individuals and/or limit extramarital sex to the greatest possible extent [9]. Third, this premise requires an attempt, not a success, to control mating access to X's spouse(s). As adultery is evident across human societies [10], this emphasizes the importance of behavioural strategies to reduce its occurrence.

Although it has been claimed repeatedly that restrictive sexual norms are virtually universal, there is a dearth of supporting evidence (but see [1,9]). Hence, I analysed whether social norms in this study's dataset entitle husbands and/or wives to at least some control over mating access to their spouse(s), or, in contrast, if both spouses are allowed to have unrestricted extramarital sex (see electronic supplementary material, appendix S1 for methodology). I found social norms that entitle mating control over spouse(s) in 100% of cultures for which data were available (survey dataset: n = 210; SCCS/EA dataset: n = 145; see electronic supplementary material, appendix S8 for full account). Cultures had diverse norms of sexual control; for instance, norms that forbid both spouses to have extramarital sex (e.g. orthodox Jews [37]); cultures where wives are required to stay faithful to their husbands, but husbands are allowed to have extramarital sex (e.g. Malekula [38]); and cultures where husbands and/or wives are allowed to have extramarital sex, but only with specific partners (e.g. Huaorani [39]). I found no culture in which social norms entitle both husbands and wives unrestricted freedom to engage in extramarital sex. These norms suggest that in virtually all human societies, group members are prevented (sometimes or always) from mating with spouses of others.

(iii) X depends on cooperation with group members that he/she prevents from mating with his/her partner(s)
Humans depend on cooperation with group members in various aspects, such as hunting and between-group competition (e.g. [31]). Furthermore, an individual's fitness critically depends on systematic alloparental care from kin and non-kin [40]. For example, the survival of parents and offspring in hunter–gatherers Hiwi (Venezuela) and Ache (Paraguay) depends on food obtained by young, unrelated males [41]; infants of the Aka people in central  Africa are carried by kin and non-kin helpers for roughly 30% of the time [42]. Humans are thus considered a communal, and even cooperatively breeding, species [40].

(iv) Summary
In conclusion, visual and audile stimuli of human mating trigger sexual arousal and sexual behaviour in both male and female observers [33,34]; across cultures, husbands and/or wives attempt to control mating access to their spouse(s) (electronic supplementary material, Appendix S8, [9]); humans live in social systems where fitness crucially depends on cooperation between group members [40,41]. I therefore suggest that the habitual concealment of legitimate mating in humans is a relatively non-costly behavioural strategy to prevent unnecessary sexual arousal in group members (proximate explanation). This simultaneously maintains control over mating access to their spouse(s) as well as cooperation with group members that are prevented from mating with their spouse(s) (ultimate explanations).

Undergraduates enrolled in introductory psychology courses: Certain misconceptions about mental illness & treatments are widely held; tend to possess weaker critical thinking skills & accept more some paranormal claims

Basterfield, C., Lilienfeld, S. O., Cautin, R. L., & Jordan, D. (2020). Mental illness misconceptions among undergraduates: Prevalence, correlates, and instructional implications. Scholarship of Teaching and Learning in Psychology, Aug 2020.

Abstract: Although several published studies have examined students’ misconceptions about psychology in general, only 1 study has focused exclusively on misconceptions about mental illness, and that study examined only 5 such misconceptions. To overcome this gap in our knowledge and to devise effective teaching strategies to disabuse college students of false information, an up-to-date survey of current misconceptions and their correlates among students is necessary. In this study, 375 undergraduates enrolled in introductory psychology courses completed an abnormal psychology misconceptions questionnaire, as well as measures assessing critical thinking, attitudes toward science, beliefs in paranormal phenomena, and vocational interests. Results revealed that certain misconceptions about mental illness and its treatment are widely held, and that compared with other students, students who endorse mental illness misconceptions tend to possess weaker critical thinking skills, are more inclined to accept paranormal claims, and are less likely to endorse scientific and behavioral views of psychology. Given the prevalence of abnormal psychology misconceptions among introductory students, we provisionally recommend assessing mental illness misconceptions early in an introductory course and utilizing empirically supported refutational methods to reduce student levels of mental illness misconceptions.

The intertemporal cortex of untrained primates can serve as a precursor of orthographic processing; the acquisition of reading in humans seems to rely on the recycling of a brain network evolved for other visual functions

The inferior temporal cortex is a potential cortical precursor of orthographic processing in untrained monkeys. Rishi Rajalingham, Kohitij Kar, Sachi Sanghavi, Stanislas Dehaene & James J. DiCarlo. Nature Communications volume 11, Article number: 3886. Aug 4 2020.

Abstract: The ability to recognize written letter strings is foundational to human reading, but the underlying neuronal mechanisms remain largely unknown. Recent behavioral research in baboons suggests that non-human primates may provide an opportunity to investigate this question. We recorded the activity of hundreds of neurons in V4 and the inferior temporal cortex (IT) while naïve macaque monkeys passively viewed images of letters, English words and non-word strings, and tested the capacity of those neuronal representations to support a battery of orthographic processing tasks. We found that simple linear read-outs of IT (but not V4) population responses achieved high performance on all tested tasks, even matching the performance and error patterns of baboons on word classification. These results show that the IT cortex of untrained primates can serve as a precursor of orthographic processing, suggesting that the acquisition of reading in humans relies on the recycling of a brain network evolved for other visual functions.

A key goal of human cognitive neuroscience is to understand how the human brain supports the ability to learn to recognize written letters and words. This question has been investigated for several decades using human neuroimaging techniques, yielding putative brain regions that may uniquely underlie orthographic abilities7,8,9. In the work presented here, we sought to investigate this issue in the primate ventral visual stream of naïve rhesus macaque monkeys. Non-human primates such as rhesus macaque monkeys have been essential to study the neuronal mechanisms underlying human visual processing, especially in the domain of object recognition where monkeys and humans exhibit remarkably similar behavior and underlying brain mechanisms, both neuroanatomical and functional13,14,15,16,39,40. Given this strong homology, and the relative recency of reading abilities in the human species, we reasoned that the high-level visual representations in the primate ventral visual stream could serve as a precursor that is recycled by developmental experience for human orthographic processing abilities. In other words, we hypothesized that the neural representations that directly underlie human orthographic processing abilities are strongly constrained by the prior evolution of the primate visual cortex, such that representations present in naïve, illiterate, non-human primates could be minimally adapted to support orthographic processing. Here, we observed that orthographic information was explicitly encoded in sampled populations of spatially distributed IT neurons in naïve, illiterate, non-human primates. Our results are consistent with the hypothesis that the population of IT neurons in each subject forms an explicit (i.e., linearly separable, as per ref. 21) representation of orthographic objects, and could serve as a common substrate for learning many visual discrimination tasks, including ones in the domain of orthographic processing.

We tested a battery of 30 orthographic tests, focusing on a word classification task (separating English words from pseudowords). This task is referred to as “lexical decision” when tested on literate subjects recognizing previously learned words (i.e., when referencing a learned lexicon). For nonliterate subjects (e.g., baboons or untrained IT decoders), word classification is the ability to identify orthographic features that distinguish between words and pseudowords and generalize to novel strings. This generalization must rely on specific visual features whose distribution differs between words and pseudowords; previous work suggests that such features may correspond to specific bigrams17, position-specific letter combinations41, or distributed visual features42. While this battery of tasks is not an exhaustive characterization of orthographic processing, we found that it has the power to distinguish between alternative hypotheses. Indeed, these tasks could not be accurately performed by linear readout decoders of the predominant input visual representation to IT (area V4) or by approximations of lower levels of the ventral visual stream, unlike many other coarse discrimination tasks (e.g., contrasting orthographic and nonorthographic stimuli). We note that the successful classifications from IT-based decoders do not necessarily imply that the brain exclusively uses IT or the same coding schemes and algorithms that we have used for decoding. Rather, the existence of this sufficient code in untrained and illiterate non-human primates suggests that the primate ventral visual stream could be minimally adapted through experience-dependent plasticity to support orthographic processing behaviors.

These results are consistent with a variant of the “neuronal recycling” theory, which posits that the features that support visual object recognition may have been coopted for written word recognition5,6,24. Specifically, this variant of the theory is that humans have inherited a pre-existing brain system (here, the ventral visual stream) from recent evolutionary ancestors, and they either inherited or evolved learning mechanisms that enable individuals to adapt the outputs of that system during their lifespan for word recognition and other core aspects of orthographic processing. Consistent with this, our results suggest that prereading children likely have a neural population representation that can readily be reused to learn invariant word recognition. Relatedly, it has been previously proposed that the initial properties of this system may explain the child’s early competence and errors in letter recognition, e.g., explaining why children tend to make left-right inversion errors by the finding that IT neurons tend to respond similarly to horizontal mirror images of objects36,37,43. Consistent with this, we here found that the representation of IT-based decoders exhibited a similar signature of left-right mirror symmetry. According to this proposal, this neural representation would become progressively shaped to support written word recognition in a specific script over the course of reading acquisition, and may also explain why all human writing systems throughout the world rely on a universal repertoire of basic shapes24. As shown in the present work, those visual features are already well encoded in the ventral visual pathway of illiterate primates, and may bias cultural evolution by determining which scripts are more easily recognizable and learnable.

A similar “neuronal recycling hypothesis” has been proposed for the number system: all primates may have inherited a pre-existing brain system (in the intraparietal sulcus) in which approximate number and other quantitative information is well encoded44,45. It has been suggested that these existing representations of numerosity may be adapted to support exact, symbolic arithmetic, and may bias the cultural evolution of numerical symbols6,46. Likewise, such representations have been found to spontaneously emerge in neural network models optimized for other visual functions47. Critically, the term “recycling,” in the narrow sense in which it was introduced, refers to such adaptations of neural mechanisms evolved for evolutionary older functions to support newer cultural functions, where the original function is not entirely lost and the underlying neural functionality constrains what the brain can most easily learn. It remains to be seen whether all instances of developmental plasticity meet this definition, or whether learning may also simply replace unused functions without recycling them48.

In addition to testing a prediction of this neuronal recycling hypothesis, we also explored the question of how orthographic stimuli are encoded in IT neurons. Decades of research has shown that IT neurons exhibit selectivity for complex visual features with remarkable tolerance to changes in viewing conditions (e.g., position, scale, and pose)19,22,23. More recent work demonstrates that the encoding properties of IT neurons, in both humans and monkeys, is best explained by the distributed complex invariant visual features of hierarchical convolutional neural network models30,49,50. Consistent with this prior work, we here found that the firing rate responses of individual neural sites in macaque IT was modulated by, but did not exhibit strong selectivity to orthographic properties, such as letters and letter positions. In other words, we did not observe precise tuning as postulated by “letter detector” neurons, but instead coarse tuning for both letter identity and position. It is possible that, over the course of learning to read, experience-dependent plasticity could fine-tune the representation of IT to reflect the statistics of printed words (e.g., single-neuron tuning for individual letters or bigrams). Moreover, such experience could alter the topographic organization to exhibit millimeter-scale spatial clusters that preferentially respond to orthographic stimuli, as have been shown in juvenile animals in the context of symbol and face recognition behaviors18,51. Together, such putative representational and topographic changes could induce a reorientation of cortical maps towards letters at the expense of other visual object categories, eventually resulting in the specialization observed in the human visual word form area (VWFA). However, our results demonstrate that, even prior to such putative changes, the initial state of IT in untrained monkeys has the capacity to support many learned orthographic discriminations.

In summary, we found that the neural population representation in IT cortex in untrained macaque monkeys is largely able, with some supervised instruction, to extract explicit representations of written letters and words. This did not have to be so—the visual representations that underlie orthographic processing could instead be largely determined over postnatal development by the experience of learning to read. In that case, the IT representation measured in untrained monkeys (or even in illiterate humans) would likely not exhibit the ability to act as a precursor of orthographic processing. Likewise, orthographic processing abilities could have been critically dependent on other brain regions, such as speech and linguistic representations, or putative flexible domain-general learning systems, that evolved well after the evolutionary divergence of humans and Old-World monkeys. Instead, we here report evidence for a precursor of orthographic processing in untrained monkeys. This finding is consistent with the hypothesis that learning rests on pre-existing neural representations which it only partially reshapes.

Rolf Degen summarizing... Among children from highly educated parents, genetic factors had a far greater effect on educational attainment than among those whose parents had low levels of education, accounting for 75% of the individual differences

How Social and Genetic Influences Contribute to Differences in Educational Success within the Family. Tina Baier. Doctoral thesis, Faculty of Sociology at Bielefeld University, Germany. July 9, 2019.

4.4 Conclusion and Discussion
This study extended previous research on gene–environment interactions for education in two crucial ways. First, we acknowledged that not only the proximate family but also the broader institutional environment can shape genetic effects on education. Second, we extended previous research that focused originally on IQ to indicators of educational success, namely educational achievement measured in school grades and educational attainment measured in years of education. Specifically, we addressed the following research questions: Do genetic effects on educational success vary across countries, and are there differences in the social stratification of genetic effects on educational success among these countries?

We selected three advanced industrialized societies for our study: Germany, Sweden, and the United States. These countries largely differ in the setup of their educational systems and represent prototypically three different types of welfare regimes, which are often used in internationally comparative social inequality research. We hypothesized that genetic influences on educational success are overall weaker in Germany and the United States than in Sweden. Furthermore, we expected that the association between parents’ socioeconomic standing and genetic effects on educational success is stronger in Germany and in the United States than in Sweden. For Germany, our hypothesis was rooted in the early tracking system and for the United States in the less extensive welfare regime.

Our study yielded three important findings: First, we found that genetic effects on years of education are smaller than genetic effects on school grades –independent of country. Hence, genes are more important for educational achievement than for educational attainment. In addition, shared environment environmental influences on educational attainment were stronger in Germany and the United States. This supports the notion of socially stratified schooling decisions that operate over and above educational achievement (Boudon 1974; Breen and Goldthorpe 1997; Erikson and Jonsson 1996). However, we did not find effects of the shared environmental influences on educational attainment in Sweden, which diverts from previous findings based on an international meta-analysis (Branigan, Mccallum, and Freese 2013).

There are three reasons that could account for conflicting results. First, our results are based on more recent birth cohorts (i.e., we studied birth cohorts for 1975–1982, while meta-analysis examined birth cohorts for 1926–1958), and previous research shows that genetic influences on education have increased among birth cohorts born in the second half of the twentieth century (Branigan, Mccallum, and Freese 2013; Heath et al. 1985). Second, the samples used in the meta-analyses were not all population based, including the sample of Sweden where the Swedish Twin Registry was used. Third, the meta-analysis did not account for assortative mating. Without such an adjustment, genetic influences tend to be underestimated, while shared environmental influences are overestimated (Freese and Jao 2017). That shared environmental influences were absent for educational attainment in Sweden indicates that educational choices are more closely related to educational achievement, which could be explained with the less selective comprehensive schooling system.

Second, we identified cross-country differences in genetic effects on educational success. Genetic effects on educational success were least pronounced in Germany, and most pronounced in Sweden. Our hypothesis on cross-country differences was therefore supported for Germany, since genetic effects were comparatively small for both indicators of educational success. For the United States, our hypothesis was only partly supported, since genetic effects on educational attainment were comparatively small, while genetic effects on educational achievement were at least as large as in Sweden.  Together, these findings supported our expectation that more egalitarian educational systems have a positive effect on the development of genetic potential for educational success and that early tracking might be an important factor for the suppression of related genetic effects. Future research should build upon our findings and focus in a more detailed manner on the impact of the tracking system. For instance the educational system in the Nordic countries changed from a tracked to a comprehensive schooling system (see for an overview on the educational reforms in Denmark, Finland, Norway, and Sweden (Gustafsson 2018)). If tracking lowers genetic effects on education, genetic effects on educations should increase after comprehensive schools were introduced. Systematic cross-countries using a culturally homogenous set of countries (“most similar case design (Lijphart 1971)) increase the generalizability of the results.

Third, we found indications for a social stratification of genetic effects in line with the Scarr–Rowe hypothesis for educational success in Germany and the United States. We did not find any evidence for a gene–environment interaction in line with the Scarr–Rowe hypotheses in Sweden. If anything, this underlines the positive impact of more egalitarian educational systems on the development of genetic effects relevant to education.  However, differences between countries are too small and not robust enough to clearly support our hypothesis. Yet, the evidence for an interaction in line with the Scarr–Rowe hypothesis for Germany is weaker than previously found using a more fine-grained measure for years of education (Baier and Lang 2019). Thus, differences in the results for Germany between this and the previous study are likely to be driven by the harmonized measure of education which comes at the cost of preciseness. For the international comparison, however, it is crucial to investigate the same measure of education in each country; otherwise, results on genetic and environmental influences can be differently affected by the way educational attainment is measured and, thus, cannot be meaningfully interpreted across countries.

It is important to note that twins’ zygosity was unknown for our sample from Sweden.  We adjusted in line with previous research for the missing information based on the assumption that same-sex and opposite-sex dizygotic twin births are equally likely (Figlio et al. 2017). This is assumption is fairly reasonable. In addition, there is no reason to believe that the distribution same-sex and opposite-sex dizygotic twin births varies by parents’ social background which would have affected our results in regards to the Scarr– Rowe hypothesis. Nonetheless, future research is needed to gain the precise estimates of genetic influences on educational success. Since some twin pairs tend to be misclassified, our adjustment can lead to an underestimation of genetic differences between monozygotic and dizygotic twins. Therefore, our results represent lower bounds of genetic influence on educational success. Hence, the overall conclusions we draw from our cross-country comparison should not be affected by this adjustment. If anything, we underestimated the role of genes in Sweden.

For the United States, our sample sizes were comparatively small, and analyses for parents’ EGP class were based on broad categorizations (i.e., EGP classes I and II versus EGP III–VII, including the non-employed). However, the Add Health data are currently the only nationally representative dataset that includes twins. Since the quality of educational institutions varies considerably among federal states, the representativeness across states is crucial for our study purposes. Nonetheless, more research for the United States is needed to test in a more fine-grained way for the social stratification of genetic influences on educational success.

In sum, our study is the first to study cross-country differences in genetic effects on educational success. We found substantial differences in genetic effects on educational success among Germany, Sweden, and the United States. An important factor that causes these cross-country differences may be rooted in the stratification of educational systems, specifically in the strictness and timing of tracking.

Males crabs benefit from grooming because females prefer males with clean claws over dirty claws but also that the time spent grooming detracts from the amount of time available for courting females

Cost of an elaborate trait: a trade-off between attracting females and maintaining a clean ornament. Erin L McCullough, Chun-Chia Chou, Patricia R Y Backwell. Behavioral Ecology, araa072, August 4 2020.

Abstract: Many sexually selected ornaments and weapons are elaborations of an animal’s outer body surface, including long feathers, colorful skin, and rigid outgrowths. The time and energy required to keep these traits clean, attractive, and in good condition for signaling may represent an important but understudied cost of bearing a sexually selected trait. Male fiddler crabs possess an enlarged and brightly colored claw that is used both as a weapon to fight with rival males and also as an ornament to court females. Here, we demonstrate that males benefit from grooming because females prefer males with clean claws over dirty claws but also that the time spent grooming detracts from the amount of time available for courting females. Males, therefore, face a temporal trade-off between attracting the attention of females and maintaining a clean claw. Our study provides rare evidence of the importance of grooming for mediating sexual interactions in an invertebrate, indicating that sexual selection has likely shaped the evolution of self-maintenance behaviors across a broad range of taxa.

Understanding Personality through Patterns of Daily Socializing: Neuroticism is shown to be associated with shorter periods of extended conversation (periods of at least 12 minutes)

Understanding Personality through Patterns of Daily Socializing: Applying Recurrence Quantification Analysis to Naturalistically Observed Intensive Longitudinal Social Interaction Data. Alexander F. Danvers  David A. Sbarra  Matthias R. Mehl. European Journal of Personality, August 3  2020.

Abstract: Ambulatory assessment methods provide a rich approach for studying daily behaviour. Too often, however, these data are analysed in terms of averages, neglecting patterning of this behaviour over time. This paper describes recurrence quantification analysis (RQA), a non‐linear time series technique for analysing dynamic systems, as a method for analysing patterns of categorical, intensive longitudinal ambulatory assessment data. We apply RQA to objectively assessed social behaviour (e.g. talking to another person) coded from the Electronically Activated Recorder. Conceptual interpretations of RQA parameters, and an analysis of Electronically Activated Recorder data in adults going through a marital separation, are provided. Using machine learning techniques to avoid model overfitting, we find that adding RQA parameters to models that include just average amount of time spent talking (a static measure) improves prediction of four Big Five personality traits: extraversion, neuroticism, conscientiousness, and openness. Our strongest results suggest that a combination of average amount of time spent talking and four RQA parameters yield an R 2 = .09 for neuroticism. Neuroticism is shown to be associated with shorter periods of extended conversation (periods of at least 12 minutes), demonstrating the utility of RQA to identify new relationships between personality and patterns of daily behaviour.

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Mobile carrying devices: The fundamental relationship between mobile containers & foresight is easily overlooked, resulting in their significance in the study of human cognitive development being largely unrecognized

Mobile containers in human cognitive evolution studies: Understudied and underrepresented. Michelle C. Langley  Thomas Suddendorf. Evolutionary Anthropology: Issues, News, and Reviews, August 3 2020.

Abstract: Mobile carrying devices—slings, bags, boxes, containers, etc.—are a ubiquitous tool form among recent human communities. So ingrained are they to our present lifeways that the fundamental relationship between mobile containers and foresight is easily overlooked, resulting in their significance in the study of human cognitive development being largely unrecognized. Exactly when this game‐changing innovation appeared and became an essential component of the human toolkit is currently unknown. Taphonomic processes are obviously a significant factor in this situation; however, we argue that these devices have also not received the attention that they deserve from human evolution researchers. Here we discuss what the current archeological evidence is for Pleistocene‐aged mobile containers and outline the various lines of evidence that they provide for the origins and development of human cognitive and cultural behavior.