16 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
have been replicated by different researchers 
(Wiederman & Allgeier, 1993) across meth -
ods that include psychophysiological record -
ings (Buss et al., 1992), and across Western 
and non-  Western cultures (Buunk, Angleit -
ner, Oubaid, & Buss, 1996). The sex differ -
ences have also been found by using cogni -
tive methods such as attention to and recall 
of cues to sexual versus emotional infidelity; 
measures of physiological distress to imag -
ining a partner committing a sexual versus 
emotional infidelity; and functional mag -
netic resonance imaging (fMRI) technology, 
which has documented different patterns of 
brain activation in response to sexual versus 
emotional infidelity (for summaries of this 
evidence, see Buss, 2008; Buss & Haselton, 
2005). All of these empirical findings sug -
gest that jealousy is a universal and univer -
sally sex-  differentiated emotional mecha -
nism that is a good candidate for inclusion 
in a personality theory of human nature.
Jealousy is clearly not the only candidate. 
Others include fear, rage, envy, disgust, sad -
ness, and others. As Ekman (1994) notes, 
the empirical evidence suggest that these 
emotional mechanisms are universal across 
human cultures, have been adaptive phylo -
genetically, produce their manifest output in 
common eliciting contexts despite individu -
al differences and cultural differences, are 
likely to be present in closely related primate 
species, and can be activated quickly, prior 
to awareness, mobilizing action designed 
to respond to specific adaptive challenges. 
There is no reason why theories of personal -
ity should fail to include these emotions as 
candidates for the core of human nature.
Parental  Motivation
From an evolutionary perspective, offspring 
are vehicles for parents. They are a means by 
which their parent’s genes get transported to 
succeeding generations. Given the supreme 
importance of offspring as genetic vehicles, 
it is reasonable to expect that selection will 
favor mechanisms in parents to ensure the 
survival and reproductive success of their 
children. Aside from the problems of mat -
ing, perhaps no other adaptive problems are 
as paramount as making sure that one’s off -
spring survive and thrive. Indeed, without 
the success of offspring, all the effort that an organism invested in mating would be 
reproductively meaningless. Evolution, in 
short, should produce a rich repertoire of 
parental mechanisms specially adapted to 
caring for offspring.
Despite the paramount importance of pa -
rental care from an evolutionary perspective, 
it has been a relatively neglected topic within 
the field of human personality psychology. 
When the evolutionary psychologists Martin 
Daly and Margo Wilson prepared a chapter 
on the topic for the 1987 Nebraska Sympo -
sium on Motivation, they scanned the 34 
previous volumes in the series in search of 
either psychological research or theories on 
parental motivation. Not a single one of the 
34 volumes contained even a paragraph on 
parental motivation (Daly & Wilson, 1995). 
And despite the widespread everyday knowl -
edge that mothers tend to love their children, 
the very phenomenon of powerful parental 
love appears to have baffled psychologists at 
a theoretical level. One prominent psycholo -
gist who has written books on the topic of 
love noted that “the needs that lead many 
of us to feel unconditional love for our chil -
dren also seem to be remarkably persistent, 
for reasons that are not at present altogether 
clear” (Sternberg, 1986, p. 133). From an 
evolutionary perspective, however, the rea -
sons for deep parental love do seem clear, or 
at least understandable. It is reasonable to 
expect that selection has designed precisely 
such psychological mechanisms—  parental 
mechanisms of motivation designed to en -
sure the survival and reproductive success 
of the invaluable vehicles that transport an 
individual’s genes into the next generation.
The evolution of parenting motivation 
has produced in humans mechanisms that 
are far from unconditional. Empirical evi -
dence suggests that parents invest more in 
children when they are higher in phenotypic 
quality, have a high probability of genetic 
relatedness to parents, and have the ability 
to convert such aid into reproduction (see 
Buss, 2000, for a summary of the empirical 
evidence). Mothers, by virtue of internal fer -
tilization, are 100% sure that the offspring 
they bear are genetically their own. Fathers 
can never be sure. This leads to the predic -
tion that maternal love, on average, will tend 
to be stronger than paternal love. Although 
there is circumstantial evidence to support 