Twin studies allow the effects of environment ('nurture') and heredity ('nature') to be investigated in humans.
In non-humans animals we can carry out controlled breeding experiments to investigate the effects of genes and environment on various traits. However this cannot be done with humans, for both ethical (it is wrong to produce children solely for the purposes of studying them) and practical reasons (it would take too long).
Twin studies involve comparing identical and non-identical twins, or identical twins separated at birth.
Identical twins have the same genes as each other, since they come from a single fertilized egg cell. Non-identical twins share only half of their genes with each other. All twins share (more or less) the same environment. So by examining how frequently certain traits occur in identical and non-identical twins we can estimate how much these traits are affected by genes.
If identical twins are separated at birth (eg for adoption) then they will have the same genes but different environments. This also allows the effects of genes to be estimated. However, such twins will have shared the same uterus and so will have been subkect to similar, though not identical, pre-birth environments. Findings The tables below display the main results from Gottesman and Shields’ study. The four categories were:
 category 1 – both the proband and co-twin had been hospitalised and diagnosed with schizophrenia
 category 2 – both had hospitalisation, but