12 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
The mind, according to this analysis, almost 
surely contains hundreds or possibly thou -
sands of specific mechanisms. Since a large 
number of different adaptive problems can -
not be solved with just a few mechanisms, 
the human mind must contain a large num -
ber of evolved psychological mechanisms.
	•The specificity, complexity, and nu -
merousness of evolved psychological mech -
anisms gives humans behavioral flexibility. 
The definition of a psychological mechanism,  
including the key components of input, deci -
sion rules, and output, highlights why adap -
tations are not rigid “instincts” that show up 
invariably in behavior. Recall the example 
of callus-  producing mechanisms that have 
evolved to protect the structures beneath the 
skin. You can design your environment so 
that you don’t experience repeated friction. 
In this case, your callus-  producing mecha -
nisms will not be activated. The activation 
of the mechanisms is dependent on contex -
tual input coming from the environment. In 
the same way, all psychological mechanisms 
require input for their activation.
The analogy of a carpenter’s toolbox pro -
vides a useful illustration of how the nu -
merousness of evolved psychological mech -
anisms, each designed to solve a specific 
problem, afford the mind with greater  flex-
ibility. The carpenter gains flexibility not by 
having one “highly general tool” that can be 
used to cut, poke, saw, screw, twist, wrench, 
plane, balance, and hammer. Instead, the 
carpenter gains flexibility by having a large 
number of highly specific tools in the tool -
box. These highly specific tools can then be 
used in many combinations that would not 
be possible with one highly “flexible” tool. 
In a similar fashion, the human mind gains 
its flexibility from having a large number of 
problem-  specific psychological mechanisms.
This leads to a conclusion that is contrary 
to our intuitions. Most people’s intuitions 
are that having a lot of innate mechanisms 
causes behavior to be rigid and inflexible. 
But just the opposite is the case. The more 
mechanisms we have, the greater the range 
of behaviors we can perform, and hence the 
greater the flexibility of behavior.
In summary, an evolutionary perspec -
tive provides some broad specifications of 
human nature. First, it suggests a nonarbi -trary foundation for describing the contents 
of human nature—  contents described by the 
evolved psychological mechanisms of hu -
mans. Second, it suggests that human nature 
is likely to be extremely complex, consisting 
of a large number of evolved psychological 
mechanisms rather than one or a few simple 
drives, motives, or goals. Third, because 
evolved psychological mechanisms tend to 
be problem-  specific and hence activated 
only in particular contexts, it suggests that 
human nature will express itself in variable 
and context-  dependent ways rather than as 
invariant impulses, as implied by some per -
sonality theories. Fourth, it suggests that 
human behavioral flexibility comes not from 
having highly general psychological mecha -
nisms but rather from having a large number 
of specific psychological mechanisms that 
are activated and concatenated in varying 
complex sequences, depending on the adap -
tive problem being confronted. These impli -
cations render the study of human nature a 
difficult and daunting task, but also one that 
is tractable.
Progress can be made sequentially and 
cumulatively by uncovering each evolved 
psychological mechanism along with the 
adaptive problem it was designed to solve. 
Ultimately, of course, the field will have to 
discover how the mechanisms are connected 
with each other, how they are sequentially 
activated depending on circumstances, how 
the activation of one can preempt or super -
sede the activation of another, and so on 
(Buss, 2008).
The Evolution of  Motives, Goals, and  Strivings
This evolutionary analysis has profound im -
plications for personality theories of human 
nature. It provides an incisive heuristic that 
guides theorists to the sorts of motives, 
goals, and strivings that commonly charac -
terize humans. At the most general level, it 
suggests that at some fundamental level of 
description, the only directional tendencies 
that can have evolved are those that histori -
cally contributed to the survival and repro -
duction of human ancestors. For a variety 
of reasons outlined by Symons (1992) and 
Tooby and Cosmides (1990a, 1990b), what 
might seem like the most obvious candidate 
for a human motive—  the goal of maximiz -
ing inclusive fitness—  cannot have evolved. 