272 Chapter 33
Children and adolescents inhabit three rather different social worlds:
the family, the classroom and the peer group. Though distinct, the three
worlds are related. Thus, children and adolescents who come from disad-
vantaged and disharmonious families are also more likely to be attending
poor schools and spending time with disruptive peers. This can make it
very hard to tell if an association is causal. For example, if individuals
from disadvantaged families are more often truants, is this because a
disadvantaged family environment directly fosters truancy, or is the family
disadvantage simply a marker for poor neighbourhood schools that foster
truancy? To complicate matters further, adverse factors cluster together
within each of the individual’s social worlds. At home, for example,
overcrowding (one particular family-based risk factor) is linked to unem-
ployment, poverty, parental mental illness and a host of other family-based
risk factors. If overcrowding is associated with delinquency, is this a direct
effect of overcrowding, or is overcrowding simply acting as a marker for
other family-based risk factors? There are research designs and statistical
techniques for trying to answer this sort of question.
Genes, shared environment and non-shared
environment
Until the 1970s, it was widely believed that genes gave people their
physical constitution, but that it was the way they were brought up that
was responsible for their psychological characteristics. It subsequently be-
came clear that both genes and environment were relevant to personality
dimensions such as extroversion–introversion, to behavioural traits such
as aggression, and to psychiatric symptoms such as depression. Much of
this improved understanding of the role of genes and environment arose
from conceptual and methodological advances in the analysis of twin and
adoption studies. What follows is a brief introduction tobehavioural genetics.
Thevarianceof a trait is a measure of how much that trait varies between
people in the population being studied.Heritabilityrefers to the proportion
of the variance explained by genetic factors. Thus, a heritability of 25% for
a particular trait means that genetic differences between people account
for a quarter of that population’s variability in the measured variable
(under the conditions prevailing at the time of the measurement). The
rest of the variance is conventionally divided between two environmental
components:
1 Shared or common environmentrefers to environmental factors that make
people living in the same family resemble one another (making al-
lowances for resemblance due to genetic similarity). For example,
poverty, damp housing or air pollution might lead anyone living in
the family (mother father, son, lodger) to become more irritable. Or
irrespective of your genetic heritage, if you are brought up in a French-
speaking household, you speak French!
2 Non-sharedorunique environmentrefers to environmental factors that
are not shared by people living together, for example when only one