I agree with Uchiyama et al. that cultural dynamics need greater attention, but I argue that an important avenue is to aim for a better understanding of cultural variability and implications for behavioral phenotypes. Cultural evolution research has made great progress in identifying mechanisms of cultural inheritance but has been less successful in understanding relatively stable cultural differences in human populations embedded in specific ecologies. More broadly, variability in phenotypes is important to parse because population-level differences might be an expression of culturally shaped behavioral responses based on a common genetic background, differential expression of individual and population-level genetic differences leading to culturally shaped phenotypes or more complex gene–culture interactions. Any observation of behavioral phenotypes within their specific ecological environment needs careful in-situ analysis of the underlying mechanistic causes (Tinbergen, Reference Tinbergen1963), measurement equivalence (Boer, Hanke, & He, Reference Boer, Hanke and He2018; Fischer & Karl, Reference Fischer and Karl2019; Fischer & Poortinga, Reference Fischer and Poortinga2018), and a rigorous exploration of gene expression (the issue of behavioral plasticity).

To use the skin cancer example, sun tan sends different social status signals depending on the economic context. Social status is tightly linked to mating success and offspring survival (Hrdy, Reference Hrdy2000; von Rueden & Jaeggi, Reference von Rueden and Jaeggi2016). In agricultural settings, darker skin relative to other community members signals low status because of sun exposure while laboring in fields; whereas lighter skin signals higher social status. Skin cancer typically manifests relatively late in life after offspring have reached reproductive age (Fontanillas et al., Reference Fontanillas, Alipanahi, Furlotte, Johnson, Wilson, Pitts and Auton2021). All else being equal, individuals with higher social status and lower sun exposure will have higher reproductive success and lower risk of skin cancer. Given low life expectancy until quite recently (World Bank, 2021), low status individuals may not reach the age when cancer manifests or other health conditions have a larger impact on lifespan compared to skin cancer.

In postindustrial settings, most individuals work in service jobs and cannot afford being exposed to sun, whereas high social status individuals can afford leisurely sun exposure, tan signals social status, and increases reproductive success. Skin cancer may reduce the lifespan of high social status individuals only, which evolutionary speaking is offset by increased reproductive success. In addition to the conceptual implications, empirically the estimation of heritability estimates would be most accurate among high status individuals in tertiary sector economies, given the higher base rates.

Switching to cognitive ability studies, both cognitive ability and academic achievement motivation (as a relevant correlate with likely different genetic architecture) differ substantively within and across cultures (Chen et al., Reference Chen, Chen, Moyzis, He, Lei, Li and Dong2013, Reference Chen, Moyzis, Lei, Chen, Dong and Chiao2015), with substantive variation because of socioeconomic and cultural value differences across schools and societies (Dekker & Fischer, Reference Dekker and Fischer2008; OECD, 2010). Obtaining good grades in specific subjects will have different life consequences depending on cultural pressures (e.g., desirable professions, socialization goals; Bond & Lun, Reference Bond and Lun2014; Chen & Stevenson, Reference Chen and Stevenson1995), job market conditions (e.g., salaries and vacancies; Sortheix, Parker, Lechner, & Schwartz, Reference Sortheix, Parker, Lechner and Schwartz2019; Vecchione et al., Reference Vecchione, Schwartz, Alessandri, Döring, Castellani and Caprara2016), and personal ambitions. Schools (and schooling systems) both within and across cultures employ diverse teaching methods, making some gene effects more or less likely to be expressed depending on what abilities or skills are salient at a particular point in time. In line with this reasoning, Samuelsson et al. (Reference Samuelsson, Byrne, Olson, Hulslander, Wadsworth, Corley and DeFries2008) showed that heritability estimates for reading and spelling scores increased from kindergarten to first grade among Scandinavian children, but not in US and Australian children (but note differences in spelling difficulty between English vs. other Germanic languages; Hambleton & Zenisky, Reference Hambleton, Zenisky, Matsumoto and van de Vijver2010). In Scandinavia, social skills are emphasized in kindergarten whereas testing relevant material is only introduced at grade 1. An emphasis on social skills is related to communitarian and egalitarian values in Scandinavian societies, as a result of long-term ecological adaptations (van de Vliert, Reference van de Vliert2009). These patterns are parsimoniously explained via shifts in the contribution of relevant genes to cognitive (information processing demands, memory, attention, etc.) versus social-behavioral phenotypes (impulse control, social skills, etc.) that are differentially matched across locations.

More broadly, the concept of intelligence is culturally molded (Friedman et al., Reference Friedman, Miyake, Corley, Young, Defries and Hewitt2006; Serpell, Reference Serpell1979; Smith, Fischer, Vignoles, & Bond, Reference Smith, Fischer, Vignoles and Bond2013; Sternberg, Reference Sternberg1985). Depending on the test and context (e.g., group vs. individual testing, familiarity with tester and testing context), obtaining high scores may require both specific cognitive and social skills during the testing situation, which has obvious implications for gene expression. Any measured phenotype is embedded within specific cultural testing environments and reflects the outcome of both cognitive and behavioral processes, which in turn are influenced by different gene systems (Chen, Moyzis, Lei, Chen, & Dong, Reference Chen, Moyzis, Lei, Chen, Dong and Chiao2015). Crucially, the interpretation of phenotypic test scores across groups depends on adequate levels of equivalence, including examination of functional equivalence (Fischer & Poortinga, Reference Fischer and Poortinga2018).

A final point about the cultural variability of phenotypes is related to the central assumption that cultural evolution is directional and increases adaptation. This can be questioned by evidence that cultural innovation is neutral (e.g., does not enhance individual or group fitness) or maladaptive for either the individual or community (e.g., shifts in health behaviors and reproductive patterns in industrial and postindustrial societies; cargo cults; social hierarchy systems). Cultural innovations such as introducing safe drinking water may lead to greater birthrates and malnutrition because of shifting behavioral strategies (Gibson & Mace, Reference Gibson and Mace2006), hence cultural innovations in one domain might be associated with complex adaptations across the behavioral repertoire of a group, with possible non-adaptive downstream effects (Laland, Reference Laland2017; Laland et al., Reference Laland, Uller, Feldman, Sterelny, Müller, Moczek and Odling-Smee2015). I already touched upon sun tans: In industrial societies cultural innovations such as tanning studies increase social status and mating chances because of social signaling, with the negative effects of skin cancer possibly being offset by late onset, often well past prime reproductive age (Fontanillas et al., Reference Fontanillas, Alipanahi, Furlotte, Johnson, Wilson, Pitts and Auton2021). We need more focused examinations of long-term cultural differences, specifically an understanding of the functionality of behavioral phenotypes within their relevant ecological environments and then link these directly to relevant (poly)genic candidate models. In summary, I agree that cultural variability matters, but understanding the interaction between culture and genetics requires more careful attention to the phenotypic expressions before firm conclusions on cultural evolution effects can be made.