Publisher Summary A temperamental bias is a biologically based susceptibility for particular feelings and actions, usually appearing during early childhood, that experiences sculpt into a large, but limited number of possible personality traits. The affective components of each bias include the ease and intensity with which particular feelings are experienced and the individual's ability to regulate them. Although most temperamental biases are the result of heritable variation in brain neurochemistry, some temperaments can be produced by prenatal events that are not strictly genetic, including season of conception and maternal infections or stressors that alter the chemical environment during gestation. Because the number of possible biological foundations for a temperament is very large, the majority of biases remain undiscovered. There are over 290000 combinations of the 12 blood types and their variants and the probability that any two people will have the same combination is 3 out of 10000. Because there are many more distinct neurochemical profiles than there are blood types, the chance of any two individuals having the same pattern of temperaments should be far less than 3 out of 10000. Newton's inverse square law explained why there are seasons; Darwinian theory provided an account of the similarities and differences among animals; the concept of “temperament” helps to explain why fraternal twins develop different personalities and why only some individuals exposed to a stressor acquire a form of psychopathology.