The evidence suggests that we are born with the physical parameters of our nervous systems programmed by genes which are impacted upon by environmental factors in utero. However, social stimuli, from the sounds of maternal voice and voices of other family members heard during gestation, to the more obvious social influences, such as instruction from parents about how to act, become increasingly important. The termeramental nature of the person can be partly accounted for by aspects of CNS function, such as reactivity and receptor sensitivity, which might predispose a person to being anxious or inquisitive, having a better or worse memory. However, there is growing evidence that these factors can also be the product of learning contingencies.

People are different because the social and environmental contexts, reinforcement contingencies and experiences they have from these differ.