Behaviour can be innate because it is genetically based and can be inherited.

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Heston (1966) – study whether schizophrenia is genetic or not with adopted children, assuming if there are any physical and behavioural similarities between offspring and parents, that is due to genetics. He compared 50 adoptees whose mothers were diagnosed with schizophrenia and 47 who weren’t. Schizophrenia is occurring in 1% of population and the same results were in those, whose mums were not diagnosed, however, around 10% of those who had genetical predisposition later developed schizophrenia. This proved that schizophrenia has a strong GENETICAL COMPONENT.

Evaluation: Rich qualitative data – 3 psychiatrists rated subjects, as well as different reports (e.g. school), however, unequal sample (50-47) and children knowing their mothers have schizophrenia à risk of self-fulfilling prophecy, no control over variables, confidentiality breach and trauma knowing they’re adopted and who their mothers are.


Money (1974) – longitudinal study on David Reimer who lost his penis during circumcision accident à Money suggested sex change, not revealing his motif is to prove that nurture determines gender identity, not nature.  David was raised as a girl Brenda, however showed masculine behaviour, causing him trouble. Later he changed his sex back to boy again. Researcher Diamond explained that psychosexual development is not determined by genitalia or upbringing, but chromosomes and hormones à GENETICALLY BASED.

Evaluation: deception, uninformed consent, no chance of withdrawal, causing physical and mental harm.


Bailey & Pillard (1991) – studied whether homosexuality is genetic with monozygotic (MZ) and dizygotic (DZ) twins. The concordance rate for DZ was 22% and for MZ twins 52%. This shows that there is some GENETIC FACTOR to why MZ twins had double the concordance rate; however, the environmental factor is believed to affect why the rate is not 100%.

Evaluation: the sample was completely male, so generalization is impossible, also researchers advertised their study in gay newspaper, seeing that only those who did enjoy attention responded and that is not a representative sample. Also, the use of survey to determine sexual orientation of another twin (one being gay), may not be completely reliable. 

Zana Katarina L. IB Biology tutor, IB Maths tutor, IB Psychology tutor

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