8 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
Evolutionary Consequences 
for Human Nature
One important consequence of a careful 
consideration of the products of evolution -
ary processes is that the core of human 
nature largely consists of adaptations. We 
address in this section the core of human 
nature from this adaptationist perspective. 
First, we argue that all species, including 
humans, have a nature that can be described 
and explained. Second, we provide a defini -
tion of evolved psychological mechanisms— 
the core units that comprise human na -
ture. Third, we explore two illustrations 
of evolved psychological mechanisms. And 
fourth, we suggest some ways in which evo -
lutionary thinking can provide a nonarbi -
trary foundation for a personality theory 
that specifies the core of human nature.
Humans Have a  Human  Nature
All species have a nature. It is part of lion’s 
nature to walk on four legs, display a large 
furry mane, hunt other animals for food, 
and live on the savannah. It is part of a but -
terfly’s nature to enter a flightless pupae 
state, wrap itself in a cocoon, and emerge to 
soar, fluttering gracefully in search of food 
and mates. It is part of a porcupine’s nature 
to defend itself with quills, a skunk’s nature 
to defend itself with a spray of acrid liquid 
smell, a stag’s nature to defend itself with 
antlers, and a turtle’s nature to defend itself 
with a shell. All species have a nature, but 
that nature is different for each species. Each 
species has faced at least somewhat unique 
selection pressures during its evolutionary 
history and has therefore evolved at least 
some unique adaptive solutions.
Humans also have a nature—  qualities that 
define us as a species—  and all psychological 
theories imply the existence of a human na -
ture. For Freud (1905/1953), human nature 
consisted of raging sexual and aggressive 
impulses. For William James (1890/1962), 
human nature consisted of dozens or hun -
dreds of instincts. Even the most ardently 
environmentalist theories, such as Skinner’s 
theory of radical behaviorism, assume that 
humans have a nature—  in this case, consist -
ing of a few highly general learning mecha -
nisms (Symons, 1987). All psychological theories require as their core a specification 
of, or fundamental premises about, human 
nature.
Since evolution by selection is the only 
known causal process that is capable of 
producing the fundamental components of 
that human nature, all psychological theo -
ries are implicitly or explicitly evolutionary 
(Symons, 1987). Although many psycholo -
gists fail to specify their assumptions about 
the evolution of human nature (hence keep -
ing those assumptions implicit), not one has 
ever proposed a psychological theory that 
has presumed some other causal process to 
be responsible for creating human nature.
If humans have a nature and if selection 
is the causal process that produced that 
nature, then the next question is: What in -
sights into human nature can be provided 
by examining our evolutionary origins? Can 
examining the process  of selection tell us 
anything about the products  of that process 
in the human case? Can evolutionary theory 
provide heuristic value,  guiding personal -
ity researchers to important domains that 
have been neglected, unexamined, or down -
played? Can it yield specific psychological 
hypotheses that are capable of being empiri -
cally tested, and hence confirmed or falsi -
fied?
Whereas the broader field of evolution -
ary biology is concerned with the evolution -
ary analysis of grandly integrated parts of 
an organism, evolutionary psychology fo -
cuses more narrowly on those parts that are 
psychological—  the analysis of the human 
mind as a collection of evolved mechanisms, 
the contexts that activate those mechanisms, 
and the behavior generated by those mecha -
nisms. And so we explore in the next sec -
tion the subclass of adaptations that make 
up the human mind—  evolved psychological 
mechanisms.
The Nature of Evolved 
Psychological  Mechanisms
By definition, every adaptation exists in the 
form that it does because it helped solve a 
specific problem of survival or reproduction 
recurrently over evolutionary history.  The 
fact that each evolved psychological mecha -
nism was shaped by a specific survival- or 
reproduction-  related problem means that 