Behavioral epigenetics is the field of study examining the role of epigenetics in shaping animal and human behavior. It seeks to explain how nurture shapes nature, where nature refers to biological heredity and nurture refers to virtually everything that occurs during the life-span (e.g., social-experience, diet and nutrition, and exposure to toxins). Behavioral epigenetics attempts to provide a framework for understanding how the expression of genes is influenced by experiences and the environment to produce individual differences in behaviour, cognition, personality, and mental health.
Epigenetic gene regulation involves changes other than to the sequence of DNA and includes changes to histones (proteins around which DNA is wrapped) and DNA methylation. These epigenetic changes can influence the growth of neurons in the developing brain as well as modify the activity of neurons in the adult brain. Together, these epigenetic changes in neuron structure and function can have a marked influence on an organism's behavior.
Background
In biology, and specifically genetics, epigenetics is the study of heritable changes in gene activity which are not caused by changes in the DNA sequence; the term can also be used to describe the study of stable, long-term alterations in the transcriptional potential of a cell that are not necessarily heritable.
Examples of mechanisms that produce such changes are DNA methylation and histone modification, each of which alters how genes are expressed without altering the underlying DNA sequence. Gene expression can be controlled through the action of repressor proteins that attach to silencer regions of the DNA.
DNA methylation turns a gene "off" – it results in the inability of genetic information to be read from DNA; removing the methyl tag can turn the gene back "on".
Histone modification changes the way that DNA is packaged into chromosomes. These changes impact how genes are expressed.
Epigenetics has a strong influence on the development of an organism and can alter the expression of individual traits. Epigenetic changes occur not only in the developing fetus, but also in individuals throughout the human life-span. Because some epigenetic modifications can be passed from one generation to the next, subsequent generations may be affected by the epigenetic changes that took place in the parents.
Discovery
The first documented example of epigenetics affecting behavior was provided by Michael Meaney and Moshe Szyf. While working at McGill University in Montréal in 2004, they discovered that the type and amount of nurturing a mother rat provides in the early weeks of the rat's infancy determines how that rat responds to stress later in life. This stress sensitivity was linked to a down-regulation in the expression of the glucocorticoid receptor in the brain. In turn, this down-regulation was found to be a consequence of the extent of methylation in the promoter region of the glucocorticoid receptor gene. Immediately after birth, Meaney and Szyf found that methyl groups repress the glucocorticoid receptor gene in all rat pups, making the gene unable to unwind from the histone in order to be transcribed, causing a decreased stress response. Nurturing behaviours from the mother rat were found to stimulate activation of stress signalling pathways that remove methyl groups from DNA. This releases the tightly wound gene, exposing it for transcription. The glucocorticoid gene is activated, resulting in lowered stress response. Rat pups that receive a less nurturing upbringing are more sensitive to stress throughout their life-span.
This pioneering work in rodents has been difficult to replicate in humans because of a general lack of availability of human brain tissue for measurement of epigenetic changes.
Research into epigenetics in psychology
Anxiety and risk-taking
In a small clinical study in humans published in 2008, epigenetic differences were linked to differences in risk-taking and reactions to stress in monozygotic twins. The study identified twins with different life paths, wherein one twin displayed risk-taking behaviours, and the other displayed risk-averse behaviours. Epigenetic differences in DNA methylation of the CpG islands proximal to the DLX1 gene correlated with the differing behavior. The authors of the twin study noted that despite the associations between epigenetic markers and differences personality traits, epigenetics cannot predict complex decision-making processes like career selection.