To study the influence of genes and the environment, use adoption studies
If children are adopted have the genes of their biological parents but an environemnt created by their adoptive parents
Adoptive children's characteristics and behaviour are measured then compared to their adoptive parents.
In adulthood the adoptive parents and the child are compared again to see whether similarities have carried on after the child has left home. (Longitudinal design)
Alternativley, this comparison can be made between the child and its biological parents.
Adoption studies are more ethical than Twin Studies to measure the role of genes (no genetic markers involved)
Heston (1966)- investigated role of genes on schizophrenia by comparing kids who had been adopted at birth beacause their mothers had schizo. with children who had mentally healthy mothers.
Results: children interviewed as adults and it was discovered:
- 10% of adults with schizo. mothers had developed it themselves
- None of the adults with non-schizo mothers deveoped it
Therefore: suggests genes play a role in schizophrenia
More valid than twin studies as none of the adults with healthy mums had it so adoption is not a casual factor
OTHER studies have discovered environmental factors which influence the probability of an adopted child developing schizo. e.g. Tierni (1992)
- adoption studies provide a direct comparison of the influences of nature vs nurture because they isolate the environement compared to gentics
- can investigate a range of variables and use various samples and methods e.g. trans-racial adoptions and meta-ananlysis. In different approches, findings are similar so techniques is more valid
- most people are not adopted so we cannot generalise the technique to the wider population
- adopted children are usually placed in families similar to their biological ones. This makes the untangling of the influences of genes and the environment difficult.
- Heston (1966) study- there may have been something different about the mothers or children which lead to the adoption so other factors may have lead to the Schizo.
- Usually use Longitudinal studies whcih take a long time and are expensive
- Longgitudinal studies also mean the participant may drop out and they may be hard to locate in adulthood