The K-factor: Individual differences in life history strategy

Abstract

Until recently, variations in life history strategy were studied exclusively at the species level. Although this domain of study has been extended to examine systematic differences in life history strategy among various human ethnic groupings, more recent evolutionary theories of human development and related behavioral genetic work imply substantial within-group individual variation in life history strategy. We constructed a latent variable model identifying a single common factor, denoted as K, which underlies a variety of otherwise disparate life history parameters. This “K-Factor” loaded 0.36 on childhood attachment to the biological father, −0.36 on childhood attachment to any non-biological father figure, 0.38 on adult romantic partner attachment, −0.51 on mating effort, −0.58 on Machiavellianism, and −0.41 on risk propensity. The bivariate correlations of the K-factor with higher-order personality factors were statistically significant, −0.24 with “Big Neuroticism” and −0.67 with “Big Psychoticism”, and approached significance, correlating 0.12, with “Big Extraversion”. The K-factor appears to be an underappreciated individual difference variable of major importance to human development.

Introduction

Individual differences in personality present a recurring, as yet unresolved, theoretical problem for Evolutionary Psychology. Some argue that individual differences are unlikely to be the result of complex adaptations, but are instead a product of selection-irrelevant genetic variation, the random effects of sexual recombination, and non-adaptive phenotypic plasticity in response to environmental differences in development (e.g. Tooby & Cosmides, 1990). Others argue that individual differences in personality are a central aspect of the “adaptive landscape” in which humans evolved (Buss, 1991). Still others argue that individual differences evolved to reduce competition among conspecifics by filling a diverse range of social and environmental niches through frequency-dependent selection in social species (Figueredo, 1995, Figueredo and King, 2001, Figueredo et al., in press, MacDonald, 1998).

We investigate the possibility that individual differences in personality are related to sets of behavioral strategies that increase individual reproductive success within a given environmental context (MacDonald, 1998). More specifically, Rushton, 1985, Rushton, 1987, Rushton, 1990, Rushton, 1995, Rushton, 2004 proposed “Differential K Theory” to help systematize individual and group differences in life histories, social behavior, and physiological functioning. Rushton (1985) suggested that the life history analyses might usefully be extended to human differences beyond those between-species differences and within-species individual differences then being examined in non-human animals. While earlier attempts were successful in explaining behavioral and biological differences at the level of human ethnic groupings, they were much less so at the level of individual differences in personality. We provide further empirical support for the hypothesis that an individual’s personality is related to an integrated life history strategy.