Nature and Nurture 273twin is knocked down by a bus (resulting in brain damage or PTSD)
or only one twin has a best friend who is a drug addict. However, as
discussed later in this chapter, the term ‘non-shared environment’ may
be somewhat misleading since it may also incorporate measurement
error and random factors in brain development.
The relative importance of genes, shared environment and non-shared
environment is usually estimated from twin or adoption studies, though
increasing use is now being made of comparisons of full-siblings, half-
siblings and step-siblings growing up in reconstituted families. Box 33.1
summarises the twin and adoption findings for three traits: a trait that
is entirely genetically determined, a trait that is entirely determined by
shared environment, and a trait entirely determined by non-shared envi-
ronment. In practice, most traits are mixtures of these.
Box 33.1Twin and adoption findings0
MZCorrelationCorrelationCorrelationA trait entirely determined by shared environmentA trait entirely determined by non-shared environmentDZ A*B A*AMZ DZ A*B A*AMZ DZ A*B A*A10101Notes:MZ=comparingMonoZygotic (‘identical’) twins
DZ=comparingDiZygotic (‘non-identical’ or ‘fraternal’) twins
A∗B=comparing earlyAdopted children withBiological parents or siblings
A∗A=comparing earlyAdopted children withAdoptive parents or siblingsAcorrelationis a measure of similarity: a value of 1.0 means the two individuals
are identical on the trait being measured, while a value of 0 means that the two
individuals are no more alike than randomly chosen members of the population.