Personality and reproductive fitness.

The relationship between reproductive success (number of biological children) and personality was explored in 1101 postmenopausal females from the Australian twin registry. The quadratic response surface relating fitness to extraversion (E) and neuroticism (N) showed a saddle point at intermediate levels of E and N. Selection was shown to be stabilizing, i.e., having an intermediate optimum, along the axis low E, low N-high E, high N and more mildly disruptive, having greater fitness in the extremes, along the axis low N, high E-high N, low E. Neither dimension of personality considered by itself showed a significant linear or quadratic relationship to reproductive success. Sections through the fitness surface, however, show selection tends to favor high neuroticism levels in introverts and low neuroticism levels in extroverts.