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After a 4-mo study period, quantitative measures of stable behavioral traits in individual rhesus monkeys correlated significantly with platelet monoamine oxidase (MAO) activity. In particular, behavioral items reflecting social activity and social contact, both agonistic and affiliative, were inversely correlated with enzyme activity. Time spent alone was positively correlated. Since platelet MAO activity is generally stable and predominantly controlled by genetic factors, it might serve as a "genetic marker" for individual differences in "normal" behaviors possibly related to differences in MAO activity in the brain and other tissues.
Redmond et al. (Thu,) studied this question.