Twin Studies:
- One way to investigate genes & if cause behaviour is to study twins.
- Monozygotic (MZ) [Identical] who share 100% of their genes as they came from the same fertilized egg
- Dizygotic (DZ) [Non-identical] who share only 50% of their genes as they came from separate eggs.
- Measure concordance rates that tell us % of cases where behaviour occurs in both twins.
- If the behaviour is genetic there would be a higher concordance rate, usually amongst MZ as most genetically alike.
- It is not expected a characteristic is totally shared between MZ twins (100% concordance not likely to be found) so higher concordance rate in MZ suggests genetic basis for a behaviour.
| Strengths | Weaknesses |
| High Validity – amount they share their DNA differs both MZ & DZ twins share their environment so there is natural control over their environmental factors – can be sure any difference in concordance not due to enviro factors
Useful – No other humans share 100% of their DNA – no other way to study genetic influences so clearly. |
Low validity – MZ twins are treated more alike than DZ twins because they are identical & share their gender too – assume genetic similarity for higher concordance rates in MZ over DZ however MZ seem to have greater enviro similarity over DZ.
Low generalisability – twin studies tend to use volunteer sampling – this is a biased sample as volunteers may all share similar characteristics. |
Adoption Studies:
- Carried out because the environment of adopted children is not the same as that of their bio families yet they have genes in common.
- g effect of genes on SZ is being carried out a researcher might want to find out whether children of a parent with SZ more likely to develop SZ than children without a family history.
- Problem is children usually have both genes & environment in common with bio parents.
- Can use adoption studies to eliminate this problem allowing them to look for a genetic link by removing the environmental influences.
| Strengths | Weaknesses |
| Useful – they are useful ways of separating genes from the environment – this means we can assume that similarities between bio parents & children are due to genetic similarities
Allow for trends of behaviour to be studied – adoption studies tend to use longitudinal method meaning they follow the same group of children over a long period of time – allow trends of behaviour to be studies that can link to genetic influences. |
Low validity – children tend to be placed in families similar to their own, so their birth family & adopted family are very closely matched – means adopting environment may be similar to environment of birth parents.
Low generalisability – only certain types of families are accepted as adopters of children – adopting families likely to be similar to one another making research less representative. |