 1. The Evolution of Human Personality 7
hypotheses of chance and incidental byprod -
uct become increasingly improbable.
Although adaptations are generally con -
sidered to be the primary products of se -
lection, they are not the only products (see 
Tooby & Cosmides, 1992; Buss et al., 1998). 
Selection also produces byproducts of adap -
tations  and a residue of noise.  Byproducts 
are characteristics that do not solve adaptive 
problems and tend not to have functional 
design. They are carried along with charac -
teristics that do have functional design, be -
cause they happen to be coupled with those 
adaptations.
Consider the light bulb. A light bulb is de -
signed to produce light. The design features 
of a light bulb—the conducting filament, the 
vacuum surrounding the filament, and the 
glass encasement—  are all tributary to the 
production of light and part of its functional 
design. Light bulbs, however, also produce 
heat. Heat is a byproduct of light produc -
tion. It is carried along not because the bulb 
was designed to produce heat, but rather be -
cause heat tends to be a reliable incidental 
consequence of light production.
A naturally occurring example of a by -
product of adaptation would be the human 
belly button. There is no evidence that the 
belly button per se helped human ancestors 
to survive or reproduce. A belly button is not 
good for catching food, detecting predators, 
avoiding snakes, locating good habitats, or 
choosing mates. It does not seem to be in -
volved directly or indirectly in the solution 
to an adaptive problem. Rather, the belly 
button is a byproduct of something that is 
an adaptation: the umbilical cord that sup -
plied nutrition to the growing fetus.
Nonselective Evolutionary  Processes
Everything is a product of evolution,  but that 
does not mean that everything is a product 
of selection . Selection is one of multiple evo -
lutionary processes, but it is often regarded 
with special privilege—  and for good reason: 
It is the only known causal process capable 
of producing complex functional design in 
organisms, including humans. Selection is 
therefore an indispensable tool for under -
standing the design  of the human mind. For 
this reason, a large focus of this chapter is on 
selection-  based conceptual tools. Nonethe -less, we do not want the reader to conflate 
“selection” with “evolution,” which consists 
of both selective and nonselective processes. 
Here, we briefly outline the respective prod -
ucts of these different evolutionary process -
es, which we describe in greater detail later 
in the chapter.
So far, we have focused on that special 
class of mutations (i.e., genetic variants) that 
are advantageous, and therefore favored by 
selection, as well as phenomena that inevi -
tably co-occur with these adaptations: by -
products. The evolutionary process of selec -
tive neutrality  yields noise : mutations that 
neither contribute to, nor detract from, the 
functional design of the organism. In self- 
reproducing systems, these neutral effects 
can be carried along and passed down to 
succeeding generations, as long as they do 
not impair the functioning of the mecha -
nisms that are adaptations. There is a third 
and final class of mutations: those mutations 
that are deleterious and therefore selected 
against. Despite their detrimental effects, 
these mutations cannot be entirely eliminat -
ed from the population, because selection 
cannot work fast enough to do so.
In summary, evolutionary processes 
produce four products—  adaptations, by -
products of adaptations, neutral noise, and 
deleterious mutations that have yet to be se -
lected out. In principle, we can analyze the 
component parts of a species and conduct 
empirical studies to determine which of 
these parts are adaptations, which are by -
products, which represent neutral noise, and 
which are best characterized as the result of 
mutation–  selection balance. Evolutionary 
scientists differ in their estimates of the rela -
tive sizes of these four categories of prod -
ucts. Some argue that even uniquely human 
qualities, such as language, are merely in -
cidental byproducts of large brains (e.g., 
Gould, 1991). Others argue that qualities 
such as language show evidence of special 
design that render it highly improbable that 
it is anything other than a well-  designed ad -
aptation for communication and conspecific 
manipulation (Pinker, 1994). It is equally 
incumbent on both sides of this argument to 
formulate hypotheses in a precise, testable, 
and potentially falsifiable manner, so that 
the different positions can be adjudicated 
empirically.