How do people develop into distinct individuals? This question is at the heart of a debate that has been keeping psychologists busy for a long time. It all boils down to a simple dichotomy: Nature or nurture?

In other words, is it your innate characteristics (“nature”) or your upbringing and experiences (“nurture”) that shape your individuality? In the past century, arguments often favored one or the other of the two extremes ­– either the nurture view, that is, the belief that differences in behavior are caused almost exclusively by our upbringing, or the nature view, which holds that individual differences are rooted in each person’s unique and pre-wired ­­­genetic code.

Nowadays, there is broad agreement that the truth lies somewhere in the middle. Genes and environment are constantly interacting with one another, and so intricately interwoven that it is insufficient to look at either in isolation. Human development may be best understood as a range of potential trajectories that eventually manifest themselves in a particular life, depending on the interactions between personal endowments and environmental opportunities.

“It is not the nominal environment, but rather your personalized life space – the way you experience and make use of your environment – that shapes your development. “

Individuality expresses itself not only in behavioral differences from person to person, but also in differences in brain structures and functions. These are due to what we refer to as brain plasticity. Genetic and environmental influences reverberate through different mechanisms of maturation and senescence that act upon each person’s potential for healthy development, and thus individual differences in both behavior and the brain are magnified with advancing age.

Behavioral geneticists have been studying the extent to which identical twins who are raised in the same family develop differently, despite sharing the same genetic makeup and environment. While identical twins resemble each other more than other siblings, clearly their behavior is not identical. What causes these behavioral differences, over and above genes and environment?

My colleagues and I wondered what would happen if we looked not only at twins, but at an entire population of genetically identical subjects: lab mice. And what if that population were living in a well-controlled, nominally identical environment that offered equal opportunities, but also the potential for individual experiences: space to explore and play, food and water, and a place to sleep? Would individuals still develop differently, or would the mice all come to exhibit identical behavior, in keeping with that specific environment and given their specific set of genes?

Using an automated tracking system, we observed the mice for three months, which is much of the lifespan of a mouse. We found pronounced individual differences that were magnified with age. Despite having identical genes and an identical environment, the mice showed differences in their behavior (e.g., how much they explored their environment) and brain structure (how plastic their brains were in response to the environment).

“By choosing the right environmental niche for ourselves, we may be able to take optimal advantage of our personal endowments.”

Our findings support the notion that beyond genes and environment, development itself constitutes a third source of individual differences. Even more intriguingly, the more the mice showed exploratory behavior (which can be regarded as a proxy for both physical activity and curiosity), the more new nerve cells developed in the dentate gyrus of the hippocampus, a part of the brain that supports spatial exploration. This is proof of a direct link between emerging individual differences in behavior and individual differences in brain plasticity.

We therefore conclude that it is not the nominal environment, but rather your personalized life space ­– the way you experience and make use of your environment – that shapes your development.

Certain physical characteristics are determined by our genes, like the number of legs we have or the color of our eyes. It may very well be that psychological characteristics like intelligence or personality are shaped by the constant interaction between our personal genetic endowments and the environments we live in. By choosing the right environmental niche for ourselves, we may be able to take optimal advantage of our personal endowments.

In the course of our development, we are exposed to a large number of risk and protective factors. A beneficial lifestyle – remaining physically active, socially engaged, and intellectually challenged – may act as a protective factor and help us to “stay on track” while maintaining cognitive and physical health and high levels of well-being far into old age.


The author would like to thank Julia Delius, Ulman Lindenberger, Myriam C. SanderElisabeth Wenger for comments on an earlier version of this article.


Freund, J.*, Brandmaier, A. M.*, Lewejohann, L., Kirste, I., Kritzler, M., Krüger, A., Sachser, N., Lindenberger, U., & Kempermann, G. (2013). Emergence of individuality in genetically identical mice. Science, 340(6133), 756-759. doi:10.1126/science.1235294

(* denotes shared first authorship)

Lindenberger (2014). Human cognitive aging: Corriger la fortune? Science, 346(6209),572-578. doi: 10.1126/science.1254403

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