Friday, March 11, 2022

Results largely suggest no association between therapists’ experience and therapy outcome

Germer, S., Weyrich, V., Bräscher, A.-K., Mütze, K., & Witthöft, M. (2022). Does practice really make perfect? A longitudinal analysis of the relationship between therapist experience and therapy outcome: A replication of Goldberg, Rousmaniere, et al. (2016). Journal of Counseling Psychology, Mar 2022.

Abstract: Experience is often regarded as a prerequisite of high performance. In the field of psychotherapy, research has yielded inconsistent results regarding the association between experience and therapy outcome. However, this research was mostly conducted cross-sectionally. A longitudinal study from the U.S. recently indicated that psychotherapists’ experience was not associated with therapy outcomes. The present study aimed at replicating Goldberg, Rousmaniere, et al. (2016) study in the German healthcare system. Using routine evaluation data of a large German university psychotherapy outpatient clinic, the effect of N = 241 therapists’ experience on the outcomes of their patients (N = 3,432) was assessed longitudinally using linear and logistic multilevel modeling. Experience was operationalized using the number of days since the first patient of a therapist as well as using the number of patients treated beforehand. Outcome criteria were defined as change in general psychopathology as well as response, remission, and early termination. Several covariates (number of sessions per case, licensure, and main diagnosis) were also examined. Across all operationalizations of experience (time since first patient and number of cases treated) and therapy outcome (change in psychopathology, response, remission, and early termination), results largely suggest no association between therapists’ experience and therapy outcome. Preliminary evidence suggests that therapists need fewer sessions to achieve the same outcomes when they gain more experience. Therapeutic experience seems to be unrelated to patients’ change in psychopathology. This lack of findings is of importance for improving postgraduate training and the quality of psychotherapy in general. 

In 71 of 72 countries, girls more likely than boys to attribute their failure to a lack of talent; this stereotype was strongest in more gender-egalitarian countries

 The stereotype that girls lack talent: A worldwide investigation. Clotilde Napp, Thomas Breda. Science Advances, Vol 8, Issue 10, Mar 9 2022. DOI: 10.1126/sciadv.abm3689

Abstract: Recent research has shown that there exist gender stereotypes that portray men as more brilliant or inherently talented than women. We provide a large-scale multinational investigation of these stereotypes and their relationship with other gender gaps. Using a survey question asked to more than 500,000 students in 72 countries, we build a measure of the stereotypes associating talent with men and show that they are present in almost all studied countries. These stereotypes are stronger among high-achieving students and in more developed or more gender-egalitarian countries. Similar patterns are observed for gender gaps in competitiveness, self-confidence, and willingness to work in an ICT (Information and Communication Technology)–related occupation. Statistical analysis suggests that these three latter gender gaps could be related to stereotypes associating talent with men. We conclude that these stereotypes should be more systematically considered as a possible explanation for the glass ceiling.


Many of the gender gaps said to be related to the glass ceiling (47) are larger in more developed or more gender-egalitarian countries, and they are also larger among high-performing students. The first pattern suggests that the glass ceiling is unlikely to disappear as countries develop or become more gender-egalitarian. The second is worrying as high-performing students are the most likely to be concerned with the glass ceiling. Moreover, for the three gender gaps studied in this paper, we can show that the patterns observed worldwide are related to our measure of gender-talent stereotypes. To better understand why, we now turn back to our measure and try to characterize what it captures exactly, and why it also exhibits a gender-equality paradox.

What do gender-talent stereotypes actually measure?

Our measure of gender-talent stereotypes is related to gender gaps in confidence for activities that are stereotyped as difficult or requiring to be talented. For example, GTS are positively correlated across countries with gender gaps in students’ feeling to be self-responsible when failing in math (r = 0.53, see table S15B). Hence, when our measure of GTS is large, girls are more likely, relative to boys, to attribute a failure in math to their own inability rather than to external factors. As math is one of the academic fields most strongly related with brilliance or raw talent (3), this result shows that GTS are related to gender gaps in self-concept for activities that are stereotyped as requiring talent.
GTS are also associated at the country level with gender gaps in students’ belief that they can understand things quickly (r = 0.45) or that they are efficient at reading a map (r = 0.64). Hence, the stronger GTS, the more girls will think (relative to boys) that they cannot be a fast-thinker or perform a task that is usually not taught and stereotyped as masculine. GTS are also associated at the country level with gender gaps in students’ belief that they will struggle to perform a difficult task. This is actually the case when the considered task is in a domain that is stereotyped as feminine (“understanding difficult texts”; r = 0.51; see table S15). GTS are therefore positively associated to gender gaps in self-concept in domains that are stereotyped as requiring talent, as well as to gender gaps in attitudes toward tasks that are labeled as requiring talent, no matter the domain in which the tasks take place.
GTS do not relate to gender gaps in self-confidence in all contexts: There is, for example, no significant cross-country correlation between GTS and gender gaps in students’ confidence that they are good readers (r = −0.12). GTS is therefore related to gender gaps in students’ beliefs that they can perform difficult tasks in reading, but not to general self-confidence in reading. This shows that GTS more specifically capture attitudes or self-concepts for activities or tasks that are stereotyped to require talent.
GTS do not capture either a lack of ambition of girls relative to boys. On average, girls expect to be working at around 30 years old in higher-status occupations than boys, even when controlling for performance or when restricting the sample to OECD countries. There is also no significant cross-country correlation between the gender gaps in students’ expected occupational status at 30 years old and GTS (table S15). Last, GTS do not reflect a lower perceived control over succeeding at school as girls agree more than boys with the assertion “if I put enough effort, I can succeed at school” (gender gap of −0.09 SD), and there is no significant cross-country correlation between the corresponding gender gap and GTS (table S15). This is in line with the 6- and 7-year-old girls by Bian et al.’s (5) study who thought that girls get better grades in school than boys. It shows that girls can at the same time internalize a lack of talent, and a higher ability to succeed at school, confirming that the two lines of research that have examined these different stereotypes [Leslie et al. (3) on the one hand and Stewart-Williams and Halsey (16) on the other hand] are not in contradiction with each other. Together, these results illustrate the specificity of GTS, which does not deal with plain ability at school but with the lack of a special aptitude, that cannot be taught.
Last, we can show that GTS is unlikely to capture a gendered tendency for personal attribution. The question we use to build GTS is about attributing failure to the lack of a personal characteristic: talent. One may be concerned that it is not the characteristic per se that matters, but rather gender differences in attributing outcomes to oneself or a personal characteristic, no matter the characteristic that is considered. To show that this is not the case, we proceed in two steps. We first consider an item in PISA2012 about math: “Doing well in math is entirely up to me.” For this item, the gender gap (still conditional on ability) is clearly in favor of boys, showing that girls do not systematically attribute outcomes to themselves. A fundamental difference, however, with the item above is that it is about attributing success (rather than failure) to oneself. One could still argue that girls/women will attribute failure more, and success less, to themselves than do boys. To discard this hypothesis, we recall that the gender gap in the item “if I put enough effort, I can succeed at school” is in favor of girls (see above), showing that girls can in some cases attribute success to themselves or a personal characteristic. Together, the comparison of the gender gaps in the items above suggests that there are no systematic gender differences in attributing success or failure to a personal characteristic, and that the tendency to do so for girls and boys depends on the characteristic at stake. When it is about “doing math,” girls are less likely than boys to attribute potential success to themselves. However, when it is about “putting effort” to succeed, then they are more likely than boys to consider they can do it.

Why is there a gender-equality paradox regarding gender-talent stereotypes? And what does it imply?

As underlined by, e.g., Ridgeway (55), sex categorization is too deeply rooted as a system of relational sense-making for people to tolerate a serious disruption, and the bringing together of men’s and women’s worlds, the weakening of traditional gender norms, and roles about education and labor force, and politics participation that takes place in wealthier and more gender-equal countries can be associated with the enhancement of new forms of gender differentiation, like gender-talent stereotypes. This general theory is in line with the findings of England (56) or Knight and Brinton (57) that show that no country has eliminated gender essentialism. For instance, in the United States, with women’s progress toward equality in the workforce and in education, the beliefs that women are less intelligent and competent than men have weakened in the past decades (58), but “these specific gender beliefs were transformed and there is now more focus on women’s lower brilliance” (35960). Broadly consistent with this argument, we find that the more women are present and entitled to be present in education, labor force, or politics, the stronger GTS are. GTS are positively related at the country level with female mean years of study, the female-to-male ratio in the labor force, the (opposite of the) date of women’s suffrage, or the opposite of the percentage of individuals agreeing with traditional values (in the World Value Survey).
A more fundamental reason that may explain why some gender essentialist norms (regarding talent or other domains) are more pronounced in wealthier and more egalitarian countries might be that these countries have also developed more emancipative, individualistic, and progressive values that give a lot of importance to self-realization and self-expression (61). These countries tend to “give citizens greater space to fall back on an old, deeply ingrained cultural frame as they try to make sense of themselves and others and organize their choices and behaviors accordingly” (55). This could explain that essentialist gender norms can be more easily internalized in these countries, as these norms will give individuals a cultural background on which they can fall back when facing the need to express their social identities. This explanation has been developed extensively by Charles and coauthors [see (226264)] to explain math attitudes and the larger extent of occupational segregation in more developed countries. We argue here that it can also apply to gender stereotypes regarding talent. Consistent with this argument, we find a large cross-country correlation between individualistic values and GTS (r = 0.68, N = 69; see the Supplementary Materials for details).
A related explanation comes from the fact that people in more developed and more individualistic countries have different beliefs regarding human intelligence. According to Rattan et al. (65), in Western countries, people tend to think that high intelligence is not universal but only granted to some (gifted) individuals, while in India or South Africa, for example, people believe that virtually everyone is born with the potential to become highly intelligent. This implies that in more developed countries, people may be generally more likely to attribute the underperformance to a lack of inherent potential, leaving more space for gender stereotypes regarding lack of talent to develop.
The existence of a gender-equality paradox for gender-talent stereotypes finally has important implications for the lively debate on the origin of the similar paradox regarding the underrepresentation of women in STEM (science, technology, engineering, and mathematics) or math-related fields. Some scholars have explained the latter paradox by the existence of deeply rooted gender differences in preferences that materialize more easily in countries where economic constraints are more limited or in countries where men and women have more similar rights [see, e.g., (23)]. Others argue that the paradox could be explained by the emergence of stronger gender norms associating math primarily with men in more developed or more gender-egalitarian countries (2061). The fact that GTS are larger in more developed countries means that girls in those countries internalize more the fact that they are less talented than boys, which is not consistent with an easier expression of girls’ and boys’ inner interests or preferences in these countries.

Final comments and policy implications

This paper shows the existence of large and widespread gender gaps in attribution of failure to a lack of talent. These gender gaps are linked to other well-known gender gaps at the country level and across students of different abilities, suggesting that self-selection patterns of girls away from competitive and prestigious careers may also be related to gender norms regarding talent. Considering gender norms regarding talent or related concepts such as brilliance or giftedness is therefore likely to be important to understand the supply-side contribution to the glass ceiling. This is both because these norms are likely to be directly related to the scarcity of women in top positions and because they are related to some of the main factors commonly put forward to explain it.
The evidence provided in the paper suggests, in particular, that exposure to cultural stereotypes about girls’ intellectual abilities and talent leads boys and girls to develop attitudes and preferences that they may not have had otherwise. In sending these messages, our culture may needlessly limit the behaviors, preferences, and career options that boys and girls consider. Gender-talent stereotypes may actually also hurt boys. For example, it may lead them to rely too much on talent and quick learning, to underestimate the role of effort over ability in the performance of particular tasks, to despise hard and in-depth study, and to abandon school work in case of failure. Consistent with this hypothesis, we obtain on PISA data that the gender gap in homework, as well as the gender gap in the belief that “Trying hard at school is important,” both at the advantage of girls, increase with GTS. The gender gap in students’ ability to sustain their performance over the course of the PISA test, also at the advantage of girls [see (66)], is again positively correlated with GTS at the country level, suggesting further that gender-talent stereotypes may hinder boys’ ability to engage in a sustained effort.
In terms of policy interventions, trying to suppress the myth of brilliance, raw talent, and creativity might require to convey the idea that talent is built through learning and effort and through trials and errors and that it is not innate and unchangeable (64). This consists of instilling a growth mindset [e.g., (35)] and viewing success as emerging from these processes rather than as depending on the amount of fixed, inherent ability one was supposedly born with. One small starting point is to be cautious when describing peers, children, or students as creative or brilliant because of the potential for bias in these descriptions. Exposing boys and girls to successful and arguably talented female role models is also likely to be a successful practical solution [see, e.g., (366768)].

Chimpanzees: Maternal lactational investment is higher for sons than for daughters

Maternal lactational investment is higher for sons in chimpanzees. Iulia Bădescu, David P. Watts, M. Anne Katzenberg & Daniel W. Sellen. Behavioral Ecology and Sociobiology volume 76, Article number: 44 (2022). Mar 10 2022.

Abstract: Maternal lactational investment can affect female reproductive rates and offspring survival in mammals and can be biased towards infants of one sex. We compared estimates of lactation effort among mothers, assessed as their potential milk contribution to age-specific infant diets (mother-infant differences in fecal stable nitrogen isotopes, δ15N), to the timing of weaning (infant age at last nursing bout) and to maternal inter-birth interval lengths for male and female infant chimpanzees (Pan troglodytes) at Ngogo, Uganda. Infant males had greater proportions of milk in their age-specific diets, indicated by higher mother-infant differences in δ15N (Generalized Estimating Equation, GEE: p < 0.01). This may mean that mothers of sons showed greater lactation effort than mothers of daughters. Infant males stopped nursing at older ages than infant females (Kaplan–Meier product limit estimate, Breslow estimator: p < 0.05). Mothers of sons showed longer interbirth intervals than mothers of daughters (GEE: p < 0.01). All three measures indicated maternal lactational investment was higher for sons. Male infants may cost mothers more to ensure infant survival than female infants because males are more vulnerable and/or because maternal genetic returns on investment are greater for sons than daughters, as male philopatry means that chimpanzee mothers can have more influence on the reproductive success of sons. Chimpanzee females may trade off growth-related benefits of high lactational investment in male offspring against reduced reproductive rates.

Significance statement: Maternal investment via lactation affects the reproductive success of female mammals and their offspring and can be biased towards infants of one sex. We investigated lactational variation among wild chimpanzees in relation to infant sex using three proxies for maternal lactational investment: fecal stable nitrogen isotopes, a physiological biomarker that may provide an estimate of lactation effort; observations of nursing, which we used to establish weaning ages; and the lengths of intervals between births of surviving infants. Chimpanzee mothers biased lactational investment toward sons on all three indicators and showed reduced fecundity due to longer inter-birth intervals for mothers of sons than for mothers of daughters. These results would be expected if greater maternal investment toward sons leads to better condition and higher reproductive success for sons later in life, thus to greater inclusive fitness for mothers.


Our results collectively indicate that chimpanzee females invest more heavily via lactation in sons than in daughters, which supports the hypothesis. Physiological biomarker data revealed that sons had greater proportions of milk in their diets than daughters, which indicated that mothers could have made greater lactation effort for sons, and that sons were weaned at later ages than daughters. Variation in age at weaning had reproductive implications for adult females. Data on inter-birth intervals showed that mothers of sons experienced more delayed fecundity than mothers of daughters. That results of statistical analyses of demographic, behavioral, physiological data were in the same direction lends support to the argument that this is a true pattern. Our results are in line with previous studies that found evidence for higher maternal investment in sons for chimpanzees at Taï (Boesch 1997; Fahy et al. 2014) and Gombe (Lonsdorf et al. 2020); in other nonhuman primates (spider monkeys, A. paniscus: McFarland Symington 1987; mountain gorillas, G. beringei: Robbins et al. 2007; Eckardt et al. 2016; Robbins and Robbins 2021); and in other mammals (African elephants, L. africana: Lee and Moss 1986; seals, M. angustirostris: Reiter et al. 1978Arctocephalus spp.: Trillmich 1986; Lunn and Arnould 1997; Iberian deer, C. e. hispanicus: Landete-Castillejos et al. 2005).

As assessed by isotopic data, the divergence in dependence on maternal milk between male and female infants was clearest early in infancy (≤ 1 year old, > 1.5 to 2 years old) and late in infancy (> 3.5 years old), when males continued relying on milk while many females were weaned (Figs. 1 and 3; when infants > 5 years old were removed from the analyses, results did not change: Supplementary Information). At some ages, however, infant females relied on milk more than males, and overlap between the mother-infant stable nitrogen isotope differences of male and female infants was considerable for all age categories. Thus, while analyses indicated that infant males relied on milk more than females overall, these results should be interpreted with caution as indications of potentially real effects that warrant further investigation to validate and replicate. Similarly, and consistent with an evolutionary conceptual framework in which infant sex is not expected to be the sole driver of maternal investment, the range of observed weaning ages and female interbirth intervals overlapped, which indicated that some sons were weaned earlier than most daughters and that some mothers of sons had shorter inter-birth intervals than most mothers of daughters. This suggests that other variables, including maternal, ecological, or social factors not measured here, also influenced developmental trajectories. For example, infant carrying is presumably the second most energetically expensive form of maternal investment in most primates (Altmann and Samuels 1992); we do not have data on age-independent variation in the time mothers carried infants, and thus cannot assess its possible contribution to variation in inter-birth intervals. Relatively long inter-birth intervals could result from poor maternal condition, as seems to be the case for olive baboons, Papio anubis (Patterson et al. 2021). However, no obvious reason exists why mothers with daughters would consistently be in poorer condition than mothers with sons. Also, Ngogo provides relatively favorable energetic conditions for chimpanzees, as indicated by data on feeding ecology (Potts et al. 2011; Watts et al. 2012ab) and C-peptide data (Emery Thompson et al. 2009). Higher mortality of first-born than later-born infants at Ngogo (Wood et al. 2017) is likely to be an effect of maternal condition that results from trade-offs that primiparas face between investing in their own growth versus maternal investment, but we found no effect of parity on weaning ages.

Male infants might have relied more on milk than non-milk foods than same-aged females because they ingested less solid food. If so, this could explain the higher mother-infant δ15N differences for males, because even if absolute milk intake was similar between the sexes, milk would still have contributed a greater proportion of the male infant diet relative to non-milk food. Infant solid food intake data at Ngogo are needed to resolve this issue. At Gombe, there were no differences in the amount of time that male and female infant chimpanzees spent feeding on solid foods up to 5 years of age (Lonsdorf et al. 2014). It may thus be that the higher mother-infant δ15N differences we found for infant males at Ngogo were due to chimpanzee mothers of sons synthesizing more milk, on average, than mothers of daughters. Also, the δ15N differences we obtained probably did not occur because mothers and infants ate different solid foods. Dependent chimpanzee offspring usually foraged at the same time and on the same foods as their mothers, and mothers always shared foods that were difficult to access or extract (Badescu et al. 20172020).

Stable nitrogen isotope ratios primarily reflect the protein component of milk. We therefore cannot exclude the possibility that total energetic lactational investment was similar for mothers of sons and daughters, but mothers of daughters made milk lower in protein and richer in fats and/or sugars compared to mothers of sons. Differences in maternal milk compositions for sons and daughters have been documented in some species (Landete-Castillejos et al. 2005; Hinde 20072009). Nevertheless, our findings that infant males are weaned later than infant females and that mothers of sons have longer IBIs are strong indications of higher total lactational costs of raising males.

In comparing physiological and behavioral assessments of weaning for the four infants for whom we could determine weaning ages isotopically, we found that behavioral and physiological weaning ages matched exactly for one male, but the second was not observed nursing during the two months before he was physiologically weaned. This disparity could have occurred because he was only night-time nursing for the last few months before he stopped drinking milk. If so, he was the only infant in our dataset who exhibited night-time nutritive nursing despite being identified as behaviorally weaned. We could only assess whether physiological and behavioral weaning ages matched for one of two females. This infant nursed during the month that she was physiologically weaned, which indicated that she was comfort nursing without milk transfer, a pattern described among several of our study infants (Bădescu et al. 2017). Because the sample of infants for whom we could establish weaning ages isotopically was so small, we could not conclusively say whether males were physiologically weaned later than females. However, this seems to be a strong possibility given that physiological weaning occurred an average of 23.5 months later for the two males than the two females for whom we had firm data.

Mothers may be under direct selective pressures to invest more in sons because infant males require greater maternal investment to survive. Infant males could also be more demanding of maternal investment and may solicit greater lactation effort from their mothers than infant females. In some highly sexually dimorphic species, infant growth is faster in males than females, and males therefore require more lactation effort (e.g., mountain gorillas, G. beringei: Eckardt et al. 2016; western gorillas, G. gorilla: Meder 1990; Leigh and Shea 1996; California sea lion, Zalophus californianus: Oftedal et al. 1987; Galápagos sea lion, Zalophus wollebaeki: Piedrahita et al. 2014). However, chimpanzee body mass dimorphism is moderate, and while growth data are not available for wild chimpanzee infants younger than 3 years old, the absence of sex differences in body mass at age 3 at Kasekela (Pusey et al. 2005) and the similarity of male and female growth trajectories there and at Kanyawara (Emery Thompson et al. 2016) imply that no consistent sex difference in infant growth exists. Second, male offspring may be biologically less resilient and more sensitive and may therefore be less buffered in the face of hardship than female offspring (“fragile male hypothesis”: Clutton-Brock et al. 1985; Stinson 1985; Lindstrom 1999; Wells 2000; Battles 2016). Higher infant mortality rates for males than females have been demonstrated in several mammals including Galápagos sea lions, Z. wollebaeki (Kraus et al. 2013), red deer, Cervus elaphus (Clutton-Brock et al. 1985), and humans, Homo sapiens (Kraemer 2000), as expected under this hypothesis. Infant mortality at Ngogo is substantial and resembles that at other chimpanzee research sites, but whether a sex difference exists is unclear because mortality is highest for very young infants, for whom determining sex is difficult (Wood et al. 2017). At Gombe, immature male chimpanzees between 10 and 15 years of age (but not at younger ages) showed higher mortality than females after maternal loss (Stanton et al. 2020); this suggests that early life mortality in chimpanzees is higher for males. Male offspring could suffer injuries more often, given that (as in other nonhuman primates), they spend more time in social play and engage in rough and tumble play more than females (Owens 1975; Forster and Cords 2005; Lonsdorf et al. 2014). Chimpanzee mothers may therefore face selective pressure to provide high maternal investment in sons, via higher lactation effort and longer lactation length, to increase the probability that sons survive, especially during times of food scarcity or infectious disease epidemics (Clutton-Brock et al. 1985; Stinson 1985; Lindstrom 1999; Wells 2000; Battles 2016). However, food scarcity probably did not affect the differential maternal investment of infants in our study, as noted above. Also, none of our study infants suffered major injuries during data collection. A respiratory virus outbreak at Ngogo in 2016–2017 led to a spike in infant mortality (Negrey et al. 2019), but we cannot assess whether a sex bias in mortality occurred because some young infants died before observers had been able to ascertain their sex.

Female chimpanzees may stand to gain greater inclusive fitness by investing more in sons than in daughters. Female mammals generally produce a limited number of offspring in their lifetimes, whereas males can theoretically produce few or no offspring, or many, depending on their condition and access to fertile females (Trivers 1972). Especially when conditions are good, such as when food availability is high, maximum potential reproductive output is much higher for males than for females (Trivers and Willard 1973; Bercovitch 2002). Similarly, mothers in good condition are predicted to bias investment towards offspring of the sex that is most likely to benefit from the increased maternal contribution (Trivers and Willard 1973; Bercovitch 2002). In male philopatric species, sons are more likely to benefit from the added maternal investment under good conditions than are daughters. At Taï, females identified as high-ranking, who were presumably in better condition than lower-ranking females because of better access to high quality resources, had longer inter-birth intervals following births of sons than births of daughters. Boesch (1997) argued that this showed they invested more via lactation in sons. In contrast, low-ranking females, who were presumably in worse condition, had longer inter-birth intervals after producing daughters and apparently made more lactational investment in daughters than sons (Boesch 1997). Compared to other chimpanzee research sites, food abundance at Ngogo is high and feeding competition is low (Potts et al. 200920112020), and female energetic condition there is probably generally good (Potts et al. 2011; Potts 2013), which helps explain why females do not form dominance hierarchies there (Wakefield 2008). All mothers in this favorable ecological context could benefit from investing more heavily in sons because the potential for high reproductive success, and hence greater inclusive fitness for the mothers, is higher for males than for females (Trivers and Willard 1973; Bercovitch 2002). However, how well the Trivers and Willard (1973) hypothesis applies to chimpanzees is still unclear. Moreover, the relatively high gregariousness of Ngogo females and relatively low variation in fruit abundance there argue against competition effects on female fecundity like those proposed for Gombe (Pusey et al. 1997) and Kanyawara (Kahlenberg et al. 2008). As Riedel et al. (2011) argue, chimpanzee socioecological circumstances are not universally the same, and care should be taken before generalizing findings from any one community to the species level.

Compared to other long-term research sites, reproductive skew among males at Ngogo is low and mean male reproductive success is high (Langergraber et al. 2017). Prior to the permanent community fission, all Ngogo males gained long-term reproductive benefits from participating in cooperative territory defense (Langergraber et al. 2017). Thus, females at Ngogo, in favorable ecological circumstances that allow a large community with many males who can do well in intergroup competition, could benefit from investing more heavily in sons because success in intergroup competition increases mean male reproductive success (Potts et al. 200920112020; Langergraber et al. 2017).

In contrast, daughters might benefit from relatively early nutritional independence. This could facilitate social independence, which is important because most chimpanzee females must establish social relationships with strangers in their new communities. In ursine red howler monkeys (Alouatta arctoidea), females who emigrated from natal groups were weaned faster than those who stayed in their natal groups (Crockett and Rudran 1987), which suggests that early weaning provides benefits associated with dispersal. However, the benefits of faster development for female chimpanzees, if any, are unclear, especially because juvenile and young adolescent females at Gombe associated more closely, not less, with their mothers than males did (Pusey 19831990). Thus, it seems unlikely that chimpanzee females acquire social competence faster than males.

Whether early maternal investment in sons influences the sons’ reproductive success in chimpanzees is unclear, partly because we do not have data on infant and juvenile growth. Bonobos (Pan paniscus) are also male philopatric, and female bonobos can influence the mating opportunities, rank, and reproductive success of their sons after weaning. Such influence seems mostly due to the effect of mother-adult son alliances on male-male competition (Surbeck et al. 20112019). Female chimpanzees rarely intervene in contests between adult males and do not directly influence their adult sons’ dominance ranks (Surbeck et al. 2019). Whether early maternal investment influences adult male chimpanzee competitive ability is unknown. At Taï, the presence of mothers was positively associated with post-weaning growth, although this applied to both offspring sexes (Samuni et al. 2020). Likewise, association between mothers and sons before maturity, but after weaning, was positively associated with male reproductive success (Crockford et al. 2020), but causality, if any, was unclear. Any causal influence is likely to act via early growth and nutrition provided by maternal milk, as happens in bighorn sheep (Festa-Bianchet et al. 2000). Adult body size presumably influences male dominance ranks in chimpanzees, and male rank and reproductive success are positively correlated (Wroblewski et al. 2009; Langergraber et al. 2013). If adult body size is also positively associated with early growth and size at weaning, high lactational investment for male offspring could indeed bring fitness payoffs (Clutton-Brock et al. 1984; Clutton-Brock 1991; Lee et al. 1991). Differential maternal investment in sons by chimpanzee females in our study may translate into better condition for male offspring later in life, which could lead to higher male reproductive success and greater inclusive fitness for mothers. Cross-sectional data showing longer inter-birth intervals following births of sons than births of daughters imply that females trade off any growth-related benefits of high lactational investment in males against reduced reproductive rates, as Emery Thompson et al. (2016) argued for chimpanzees at Kanyawara. Continued documentation of within-female variation in inter-birth intervals as a function of offspring sex will help address this issue.