10 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
same-sex rivals or combat other threats to 
a valued romantic relationship (Buss, 2000; 
Buss, Larsen, Westen, & Semmelroth, 1992; 
Daly et al., 1982; Symons, 1979).
Stating that the output of a psychologi -
cal mechanism leads to solutions to specific 
adaptive problems does not imply that the 
solutions will always be optimal or invari -
ably successful. A rival may not be deterred 
by your threats. Your partner may have a 
fling with your rival despite your display 
of jealousy. The main point is not  that the 
output of a psychological mechanism always  
leads to a successful solution, but rather that 
the output of the mechanism on average  
tended to solve the adaptive problem in the 
environment in which it evolved better than 
outputs from alternative designs present in 
the population during those periods.
Moreover, evolved psychological mecha -
nisms such as sexual jealousy adaptations, 
“are hypothesized to lie dormant until they 
are activated by cues signaling that [the] 
adaptive problem is being confronted” (Buss 
& Shackelford, 1997, p. 348); that is, we 
should not expect manifest  sexual jealousy 
unless the adaptations responsible for pro -
ducing these outputs—  the universal mecha -
nisms —are activated by input signaling the 
adaptive problem. This central property of 
psychological mechanisms—  that their out -
put is dependent on environmental input—
has the potential to unify propositions of 
human nature with individual differences, 
and is a point we take up later in the chapter.
Describing the Human Mind: 
Important Properties of Evolved 
Psychological  Mechanisms
We examine in this section several important 
implications for understanding the human 
mind based on the properties of evolved psy -
chological mechanisms.
	•Evolved psychological mechanisms 
provide nonarbitrary criteria for “carving 
the mind at its joints.” A central premise 
of evolutionary psychology is that the main 
nonarbitrary way to identify, describe, and 
understand psychological mechanisms is 
to articulate their functions—  the specific 
adaptive problems they were designed by se -
lection to solve.Consider the human body. In principle, 
the mechanisms of the body could be de -
scribed in an infinite number of ways. Why 
do anatomists identify as separate mecha -
nisms the liver, the heart, the hand, the 
nose, and the eyes? What makes these divi -
sions nonarbitrary compared with alterna -
tive ways of dividing the human body? The 
answer is function. The liver is recognized 
as a mechanism that performs functions dif -
ferent from those performed by the heart or 
hand. The eyes and the nose, although they 
are close to each other on the face, perform 
different functions and operate according 
to different input (electromagnetic waves in 
the visual spectrum vs. odors). If an anato -
mist tried to lump the eyes and the nose into 
one category, it would be seen as ludicrous. 
Understanding the component parts of the 
body requires the identification of function.
	•Function provides the only sensible 
nonarbitrary way to understand these com -
ponent parts.  For the same reason, a power -
ful nonarbitrary analysis of the human mind 
is one that rests on function. If two compo -
nents of the mind perform different func -
tions, then they can be regarded as separate 
mechanisms (although they may interact 
with other mechanisms in interesting ways).
	•Evolved psychological mechanisms 
tend to be problem-  specific. Imagine giving 
directions to someone to get from New York 
City to a specific street address in San Fran -
cisco, California. If you gave some general 
directions such as “head west,” the person 
might end up as far south as Texas or as 
far north as Alaska. The general direction 
would not reliably get the person to the right 
state.
Now let’s suppose that the person did get 
to the right state. The “go west” direction 
now would be entirely useless, since west 
of California is ocean. The general direc -
tion would not provide any guidance to get 
to the right city within California, let alone 
the right street address. To get to the right 
state, city, street, and location on that street, 
you need to give more specific instructions. 
Furthermore, although there are many ways 
to get to a particular street address, some 
paths will be far more efficient and time sav -
ing than others.
The search for a specific street address 
across country is a good analogy for what is 