Research Scientist at the CNRS, Paris and Associate Professor at ENS, Paris, France
Phenotypic Plasticity

Humans all over the world share the same genome, the same neural architecture and the same behavioral niche (three-generational system of resource provisioning, long-term pair-bonding between men and women, high levels of cooperation between kin and non-kin). At the same time, human cultures are highly variable. Some societies see revenge as a duty, others as a sin; some regard sex as a pleasure, others as a danger, and some reward innovations while others prefer traditions. In many instances, these cultural differences are robust, and can last for millennia despite cultural contacts, political assimilation or linguistic replacement.

How can we account for such variability? Traditionally, it is assumed that cultural variability cannot be explained by species-specific evolved mechanisms and that it must be the product of socially transmitted norms in the forms of religious beliefs, informal enforcement or political conquest.

This assumption is based on a common misconception about natural selection, which is wrongly thought to select mechanisms that systematically produce universal, uniform and unchanging behaviors. But all evolved mechanisms, physiological or psychological, actually come with a certain level of flexibility in response to local contexts. This is called phenotypic plasticity. The genotype codes for a mechanism that is able to express different phenotypes (organs, behaviors) in response to detectable and recurring changes in the environment.

Tanning is a case in point. While the mechanism of skin pigmentation is a universal adaptation to protect human cells from ultraviolet damage and to synthesize vitamin D, it responds differently to different contexts, making skin darker in low latitudes (and in the summer) and lighter in high latitudes (and in the winter).

Phenotypic plasticity has been shown to be evolutionary advantageous when there are different optimal phenotypes in different environments. This is of course the case for tanning: the optimal level of tanning differs whether an individual lives in high or low latitudes and whether it is winter or summer. Natural selection must therefore be able to maintain the right skin color despite variations in the environment. One solution is to select for a certain level of plasticity (skin pigmentation is not completely plastic) and to build a mechanism that can detect the amount of light, and adjust the level of melatonin accordingly.

The study of phenotypic plasticity has long been limited to physical traits (such as skin pigmentation). However, recent research in neuroscience, ecology and psychology has shown that phenotypic plasticity extends to behaviors. For instance, in a harsh and unpredictable environment where the future is dreary, organisms tend to adopt a short-term life strategy: maturing and reproducing earlier, investing less in offspring and in pair-bonding, being more impulsive. On the contrary, in a more favorable and predictable environment, organisms switch to a long-term strategy: maturing and reproducing later, investing in offspring and in pair-bonding, and being more patient. Importantly, these switches between present-oriented and future-oriented behaviors can affect all kind of behaviors: reproduction and growth of course, but also attitudes toward consumption, investment in learning or health, trust in others, political opinions, technological innovation, etc. In fact, every behavior (and neural structure) for which time and risk are relevant dimensions is likely to involve a certain degree of plasticity.

Could phenotypic plasticity be relevant to explain cultural differences?

When we observe cultural differences between two societies, we can’t help but think that the difference has its roots in different cultural heritages (religious, legal, literary, etc.). This is because we have no alternative mechanism to explain such differences. Suggesting that the two groups differ in terms of psychological mechanisms seems to mean going back to the 19th century "national character" studies or to mysterious "cultural mindset" that re-describe the phenomenon rather than explain it. By contrast, phenotypic plasticity offers a plausible (and not mutually incompatible) mechanism to explain why people have different mindsets in different societies, why they are more impulsive, why they trust others less or why they are afraid of innovations. Even better, it makes some predictions as to how the environment should impact people’s psychology. For instance, a common sense idea is that people should innovate when they are in danger and need an urgent solution. Evolutionary theory suggests the opposite: when resources are scarce and unpredictable, innovation is too risky: Better stick to what you know than jeopardize everything. And indeed, this is what people do.

Finally, phenotypic plasticity has the potential to solve several limitations of the standard culture-as-transmission-of-information paradigm: Why does the same cultural background (language, religion, ethnicity) give rise to radically different behaviors according to whether an individual was born in a low vs. a high social class or in an old vs. recent generation? Or how does an old and apparently robust cultural phenomenon crumble in a few generations, sometimes in a few years, and without any cultural external input? This may be because different environments triggered different "behavioral strategies" in people, transforming a common cultural heritage into diverging new cultures.

To sum up, phenotypic plasticity is key in the study of human behavior. It provides a framework to account for the fact that the same genome and the same neural architecture can give rise to cultural variability in humans.