Nature and Nurture 279irrespective of usual variations in the environment, as appears to be the
case for autism. Second, genetic liability may confer greater susceptibility
to less favourable environments, for example, a child with a genetically
irritable temperament may be more prone to have tantrums and develop
behavioural symptoms in response to insensitive parenting. A recently
documented example of this phenomenon is that children with one
variant of the monoamine oxidase A gene are more prone to develop
antisocial behaviour if their parents are relatively harsh, but not otherwise.
Third, genetic liability may predispose an individual to seek out or end up
in a context that has a higher exposure to known risk factors, for example,
youths with conduct disorder are more likely to take drugs and alcohol.
Epigenetics and the environmental modulation
of gene expression
Epigenetics refers to changes in gene expression that arenotbrought about
by changes in the underlying DNA sequence. The crucial importance of
epigenetic processes is evident from the combination of two facts. Firstly,
nearly all of an individual’s healthy cells have the same DNA (though there
are exceptions such as the variation in B lymphocyte DNA brought about
by the somatic hypermutation that increases antibody diversity). Secondly,
our very existence as multi-cellular organisms made up of different types
of cells depends on different profiles of gene activation in each cell type.
Conjuring cellular diversity out of genetic identity involves epigenetic
rather than genetic processes.
What epigenetic processes are involved? In some instances, changes
in gene expression are brought about by adding methyl groups to the
DNA itself, or by removing these again; these changes to do not alter the
‘genetic code’ that is stored in the sequence of base pairs along the DNA
double helix. In other instances, epigenetic modification involves chemical
changes to the histone proteins that are associated with DNA.
The epigenetic processes that bring about the differentiation of a single-
celled zygote into many cell types have more to do with the embryo’s
internal logic than environmental influences. Nevertheless, fairly similar
processes have been shown to be involved in the ways that the environ-
ment – including the social environment – can induce lasting changes in
the brain, thereby affecting behaviour and physiology in a way that seems
very relevant to child and adolescent psychiatry.
One particularly thoroughly researched example is the way early ma-
ternal care can have a lasting effect on rats’ stress responses. Rat mothers
vary in how much they lick and groom their pups, and ‘high licked’ pups
grow up to respond less markedly to stress than do ‘low licked’ pups:
when exposed to stress, low-licked pups show more fearfulness in their
behaviour, and their hypothalamic–pituitary–adrenal response is more
extreme. But, we hope you will ask, couldn’t that be due to genetic effects
or reverse causality? Good questions! In principle, it could be that there are
one or more genes that promote pup-licking in the mother – with those