THE ROUGH GUIDE TO PSYCHOLOGY
Consider the gene that codes for brain-derived neurotrophic factor
(BDNF) – a protein that encourages neuron growth. There are two possible
versions of this gene, one containing the amino-acid methionine, the
other containing the amino-acid valine. Each person carries either one of
each, or two copies of just one of the versions (known as val-mets, val-vals,
or met-mets, respectively). Crucially, the permutation you have is strongly
linked to the personality dimension of neuroticism (see p.178). Met-mets
tend to be the most neurotic of all, suffering from more depression and
anxiety. Val-mets, in turn, tend to be more neurotic than val-vals.
On its own, variation in the gene for BDNF only accounts for about
four percent of the variation in people’s neuroticism, so other genes
and the environment must also be involved. The overall influence
of genes on any given characteristic is known as that characteristic’s
heritability index. Genes explain about sixty to eighty percent of varia-
tion in height, for example, and about fifty percent of the variation in
personality. However, it’s important to treat these kinds of estimates
with caution. Whenever a study cites the relative contributions of genes
and the environment, this pertains to the particular sample under
investigation at that particular time in those particular circumstances.
The larger, more varied and widely distributed the sample, the more
trustworthy the heritability estimate.
Danish identical twins taking part in a study of the genetic and environmental
factors that affect ageing. The Danish Twin Registry started recruiting in 1954 and
by 2005 contained more than 75,000 twin pairs born between 1870 and 2004.