The idea of continua of reactivity was taken up by
Kevin MacDonald (1995). MacDonald proposed that the
normal range of observed variation on personality dimensions
represents a continuum of viable alternative strategies
for maximizing fitness. In this view, average fitness would
be about equal across the normal range of any given personality
dimension, but individuals of different personality
levels might differ in the way that they achieved their
fitness—for example, by investing in reproductive rather
than parental effort. Implicit in MacDonald’s formulation,
but perhaps not examined in enough detail, is the concept
of trade-offs. The idea of trade-offs is reviewed in detail
below, but the key point is that if two levels of a trait have
roughly equal fitness overall and if increasing the trait
increases some component of fitness, then it must also
decrease other components. Every benefit produced by
increasing a trait must also produce a cost. If this is not the
case, there is no trade-off, and natural selection is directional
toward the higher value of the trait.