Personality is subject to biological as well as psychological influences. Mind and body are inextricably united. Temperament refers to biologically based differences in personality, often evidenced as emotional reactivity to new or potentially frightening stimuli. It is the basis, for example, of one person's shyness and another's bold adventurousness. Allport accepted the empirical research available in his day indicating that temperament constitutes an inherited biological foundation for personality. Since that time, additional evidence further supports the importance of biologically based temperament, already observable in infancy (e.g., Kagan, 1989; Kagan & Snidman, 1991a, 1991b). Allport (1937b) listed inherited physique and intelligence, together with temperament, as "the three principal raw materials of personality" (p. 107).
How important is heredity as a determinant of personality? Allport asserted that it is always important. "No feature of personality is devoid of hereditary influences" (Allport, 1937b, p. 105; emphasis in original). All are influenced by experience, too. Allport offered a mathematical expression of this pervasive influence of heredity through a multiplicative equation:
Personality =/(Heredity) X (Environment)
"The two causal factors are not added together, but are interrelated as multiplier and multiplicand. If either were zero there could be no personality," he stated (p. 106). The mathematical properties of an alternative, additive model (which adds heredity and environment instead of multiplying them) would be different since then heredity and environment could have independent effects, and either could be zero without negating the effect of the other.
This was a theoretical statement, based on reasoning rather than research, at this early time in the history of personality as an academic area. Allport anticipated later biological and medical research to understand biological contributions to personality. Allport commented, "I believe . . . we'll never have a complete psychology of personality until we have a much better knowledge of genetic factors" (Evans, 1981b, p. 49).
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