4 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
differences are systematically linked. Over 
the past few decades, however, the field of 
personality psychology has retreated some -
what from this grand goal. Most empirical 
research on personality psychology deals 
with individual differences, not with human 
nature, let alone with the links between 
human nature and individual differences 
(e.g., McCrae & John, 1992). By focusing 
almost exclusively on individual differenc -
es, modern personality psychology has im -
plicitly ceded the study of human nature to 
other branches of psychology, such as social 
psychology, which typically concentrates on 
shared characteristics of human psychology 
and neglects the individual. This restricted 
focus on individual differences has led some 
to identify this as a defining feature of mod -
ern personality psychology: “that branch of 
psychology which is concerned with pro -
viding a systematic account of the ways in 
which individuals differ from one another” 
(Wiggins, 1979, p. 395). Grand theories of 
human nature that once characterized the 
field of personality psychology are now re -
garded primarily as relics of historical inter -
est.
One of our central arguments in this 
chapter is that the grand goals of personality 
psychology should be reclaimed. An analogy 
might help to clarify this argument. Suppose 
that invisible Martian scientists were as -
signed the task of studying the large metallic 
vehicles that earthlings use for transporta -
tion. One group of scientists was assigned 
the task of figuring out “car nature,” the 
basic design that is common to all cars. 
They might conclude that all cars have tires, 
a steering mechanism, a method of braking, 
and an engine that provides propulsion. The 
second group of scientists, in contrast, was 
assigned the task of determining the major 
ways in which cars differ from one another. 
They might conclude that some have large 
tires, others small tires; some have rack-and- 
pinion steering, others do not; some have 
anti-lock brakes, others do not; some have 
a six-  cylinder engine, while others make do 
with four cylinders.
The second group of scientists is faced 
with the task of determining which indi -
vidual differences are the really important 
ones, and separating these from the trivial 
ones. Cars differ in the color of the engine wires, for example, but are these differ -
ences really important (Tooby & Cosmides, 
1990a, 1990b)? The Martian scientists who 
study car differences conclude that they can -
not really accomplish their task—  identifying 
in a nonarbitrary fashion the most impor -
tant individual differences—  without talk -
ing to the first group of scientists, who are 
responsible for developing a theory of “car 
nature.” From a collaboration between these 
two groups, they arrive at a nonarbitrary 
criterion for determining which car differ -
ences are really important: those differences 
that affect the functioning of the car. Using 
this criterion, they decide that “differences 
in the colors of the engine wires” is really 
a trivial dimension of individual differ -
ences, since these differences have no im -
pact on the functioning of the car’s engine. 
But they include differences in the number 
of engine cylinders, tire size, and breaking 
mechanism, because these differences have 
a profound effect on the car’s functioning. 
The key point is that it would be illogical 
to have two entirely separate “car theorists,” 
one dealing with car nature and one dealing 
with car differences. The two are inextrica -
bly bound, and their integration is essential 
for a nonarbitrary theory for both car na -
ture and car differences.
People are not cars, and the analogy 
breaks down at a certain point. But the cen -
tral point of the analogy may be expressed 
as a syllogism: (1) If humans have a human 
nature, and (2) if the components of that 
nature were “designed” to perform certain 
functions, then (3) a nonarbitrary means for 
identifying the most important individual 
differences involves discovering those dif -
ferences that influence those functions. In 
order to establish the veracity of the first 
two premises, we must examine the causal 
processes that “designed” humans and ask 
whether this process has produced a human 
nature with an identifiable set of functions.
Selection as a Designer 
of Functional  Organisms
Scientists over the past two centuries have 
proposed a delimited set of theories about 
the causal process responsible for the design 
of humans and other life forms. One theory 