Separate studies on adults, including those in suicidology and another regarding personality in the general population, have indicated associations with their season of birth. We analyse each of these studies by multiple nonlinear regression employing a cosine function for the month of birth, and compare these studies regarding the birth months giving the maxima and minima. The method of suicide in suicide studies shows a significant month-of-birth variation similar to that for the serotonin metabolite 5-HIAA in the separate study on cerebrospinal fluid, with a peak around the birth month September and a nadir around birth in March. When comparing the personality study with the study on cerebrospinal fluid, the trait novelty seeking varies similar to that for the dopamine metabolite HVA or the norepinephrine metabolite MHPG, and the trait reward dependence varies similar to that for HVA. The trait self-transcendence varies similar to the ratio of the dopamine and serotonin metabolites. Dopamine turnover in adults thus shows a peak around the birth months November-December, and a nadir around the birth months May-June, suggesting a possible involvement of the length of photoperiod during their perinatal period. These results provide strong evidence for the influence of season of birth on adult monoamine neurotransmitter turnover, and give further support for the monoaminergic modulation of the temperament and character traits.