Genetic diversity – that is, the content of genes themselves – already makes each of us unique. But what distinguishes different cells with the same DNA is gene expression: which genes are "active" and when. Gene expression is what allows your skin cells to look and function in an entirely different way from nerve cells, and it's how all the tissues in your body can develop from the single cell of an embryo.

Now, though, scientists are starting to understand just how much environment can impact gene expression, and how experiences that alter gene expression early in life can have lasting impacts into adulthood. Gene expression could link "nature" and "nurture" in surprising (and still mysterious) ways.

Your cells need to be able to turn genes on and off to behave properly. Take a nerve stem cell, which will ultimately will develop into a neuron, or nerve cells. It needs to suppress genes that keep the cell in a "stem-like" state, and start expressing genes needed to become more nerve-like. This process, called neurogenesis, occurs throughout your embryonic development and into adulthood (in a process called, you guessed it, adult neurogenesis).

Some changes in gene expression seem to be "hard-wired" and start occurring almost immediately during development. Other biological factors enter in as well. Your hormone levels, for example, can affect how your genes are expressed. That's important for developing into a male or female in utero, and it also impacts puberty, hair growth, fertility and other factors throughout life.

Changes in gene expression could also be driven by exposure to chemicals in your environment. For example, exposure to mutagens (chemicals that cause genetic mutations) can increase or decrease how much a gene is expressed, and the resulting abnormal changes in gene expression are linked to diseases like cancer. Exposure to alcohol, for example, can trigger changes in gene expression that affect nutritional status, contributing to nutrient deficiencies. And inherited genetic mutations could also increase or decrease the expression of your genes.

As for how your cells control gene expression, there are a few ways to increase or decrease expression. One key is DNA methylation, a way of suppressing genes. The more methylated a gene is, the less it can be expressed; conversely, demethylation increases DNA expression. References 1 and 2 have good background information on this.

It turns out, life experiences can impact your gene expression as well. And new research suggests that early childhood experiences, even ones too early to remember, could affect your brain for the rest of your life.

New research published in Science looked at how mothering style affected the brain of developing mice to get at the question of how nature and nurture can collide to influence behavior. The basis of the experiment was simple: observe the mothering style of different mice, and then see how different mothering styles (attentive, neglectful) would affect the expression of a gene, called L1, in the emotional center of the offspring's brain. To help rule out genetic differences (because, remember, inherited genes can influence gene expression) the scientists also swapped out part of the litter, so a pup from a neglectful mother would be raised by an attentive one, or vice versa.

The researchers found that mice raised by an attentive mother had less methylation on their L1 gene – in other words, the gene was less suppressed – than mice raised by neglectful ones. That was true even in the litter-swapped mice, suggesting that the level of methylation (the level of gene suppression) was related to the mice's upbringing, rather than an inherited genetic factor.

These results mirror what scientists have previously seen in children – that kids who have been neglected in childhood have different methylation patterns than kids raised by attentive parents. But the research is still early, and the authors of the mice study aren't sure whether those changes in L1 methylation are also linked to changes in cognitive function, or any other neurological or psychological problems.

However, understanding how these differences in methylation develop, and which genes are most crucial to watch for, will help us better understand how nature and nurture interact to affect our behavior. And this could, one day, help doctors more effectively treat mental health issues that can stem from neglect.