 1. The Evolution of Human Personality 9
the form of the mechanism, its set of design 
features,  is like a key made to fit a particular 
lock (Tooby & Cosmides, 1992). Just as the 
shape of the key must be coordinated to fit 
the internal features of the lock, the shape of 
the design features of a psychological mech -
anism must be coordinated with the features 
required to solve an adaptive problem of sur -
vival or reproduction. Consequently, each 
adaptation—  whether physiological, mor -
phological, or psychological—  is designed 
(1) to be activated by input signaling that the 
adaptive problem it evolved to solve is being 
faced, and (2) to produce output that helped 
solve that problem in ancestral environ -
ments. For example, our callus-  producing 
adaptations are mechanisms that evolved to 
solve the problem of potential tissue damage 
caused by repeated friction (input) by pro -
ducing additional cell layers (output) that 
protect underlying tissues in response to this 
friction. Here, we elaborate on these prop -
erties of adaptations as they apply to psy -
chological adaptations, or evolved psycho -
logical mechanisms  (see Buss, 1991, 1995a, 
1995b, 2008; Tooby & Cosmides, 1992):
	•The input of an evolved psychological 
mechanism tells an organism the particular 
adaptive problem it is facing. The input of 
seeing a slithering snake tells you that you 
are confronting a particular survival prob -
lem, namely, physical damage and perhaps 
death if bitten. The differing smells of poten -
tially edible objects—  rancid and rotting ver -
sus sweet and fragrant—  tell you that you are 
facing an adaptive survival problem of food 
selection. The input, in short, lets the organ -
ism know the adaptive problem with which 
it is dealing. This occurs almost invariably 
out of consciousness. Humans do not smell 
a pizza baking in the oven and think, “Aha, 
I am facing an adaptive problem of food se -
lection!” Instead, the smell unconsciously 
triggers food selection mechanisms, and no 
consciousness or awareness of the adaptive 
problem is necessary.
Additionally, the fact that adaptations 
are not activated and do not produce their 
output in the absence of cues linked to the 
relevant adaptive problem is a key point. 
Witnessing one’s romantic partner interact -
ing intimately with a member of the opposite 
sex does not activate snake avoidance, food selection, or callus-  producing adaptations. It 
activates a different adaptation instead: the 
evolved psychological mechanisms that pro -
duce the constellation of cognitive, affective, 
and behavioral outputs that we call sexual 
jealousy.
Imagine that you go to a party with your 
romantic partner and briefly leave the room 
to get a drink. When you come back, you 
see your romantic partner seated very close 
to another member of the opposite sex (i.e., 
a mating rival) in a secluded corner of the 
room. The two are laughing and smiling at 
each other, and then pause to look deeply 
into each other’s eyes. You did not notice 
this before, but you now see that the other 
person’s hand is on your partner’s thigh, and 
your partner is touching the other person as 
well.
Although the cues present in this situation 
do not indicate with certainty the adaptive 
problem of losing your mate, several of these 
cues are statistically predictive of the threat 
of your mate being unfaithful or otherwise 
defecting (Shackelford & Buss, 1997). These 
are cues to potential infidelity, which would 
have been costly for both ancestral men and 
women. Consequently, selection would have 
favored the evolution of anti-  infidelity adap -
tations in both sexes, including psychologi -
cal adaptations designed to detect these cues 
to infidelity (input).
	•The output of an evolved psychological 
mechanism can be physiological activity, 
information to other psychological mecha -
nisms, or manifest behavior —all directed 
toward the solution of a specific adaptive 
problem.  Just as the cues to a partner’s po -
tential infidelity signal the presence of an 
adaptive problem, the output of the sexual 
jealousy mechanism is geared toward solv -
ing that problem. The outputs  of sexual jeal -
ousy mechanisms are a coordinated suite of 
cognitions, emotions, and behaviors. This 
can include gathering further information 
about the potential threat (Goetz, Shackel -
ford, Romero, Kaighobadi, & Miner, 2008; 
Schützwohl, 2006, 2008); experiencing neg -
ative emotions in response to one’s mate’s 
interactions with potential mate poachers 
(Buss, 1989a, 1989b, 2000); and control -
ling behaviors or aggressive responses (Daly, 
Wilson, & Weghorst, 1982) to fend off these 