frequencies in breeding populations change from generation to generation
because environments affect differentially individuals' reproductive success.
The next generation more closely resembles the successful breeders and
nurturers of the preceding generation, because their genes are more
frequent in the offspring generation. Generational changes in gene distri-
butions lead to changes in behavioral phenotype distributions across
species' histories, because different phenotypes arise from different geno-
types. Selection acts at the level of individual phenotypes, which more and
less contribute to the next generation, and thereby indirectly affects gene
distributions. If there were no genetic variation, there could be no evolu-
tion.

The role of evolution in the study of human development seems
confusing to many psychologists. Today's psychology is struggling to escape
its impoverished past -- 50 years of naive behaviorism, built on a theory of
deprivation effects, spelled out in unlikely scenarios. If an experimental
manipulation, usually a deprivation of normal, species-typical experience
(e.g., patterned visual experience or maternal rearing) could be shown to
prevent normal development (e.g., vision or social behavior), then the
environmental control of such developmental patterns was inferred. The
fact that normal members of the species (e.g., cats or rhesus macaques)
would have universal access to those experiences (e.g., patterned visual
stimuli or maternal contact) did not deter psychologists from explaining
normal human variation in terms of the effects of their deprivation
experiments on other species. This grand non sequitur dominated psy-
chology during the 20th century. Exclusive attention to proximal mecha-
nisms of development, based on manipulation of deprivations of normal
experience, has blinded developmental theory to understanding the history
and nature of development, human and otherwise.

Evolutionary psychology makes different assumptions and therefore asks
different questions. It assumes a human species history that shaped human
learning patterns and emotional expressions. It assumes the presence of
genetic variability in the most basic human traits, or else evolution could
not have occurred and individuals would not differ. It assumes that those
experiences that are required for species-normal development are widely
available to biologically normal members of the species. If failure to
encounter experiences that are essential for normal sensory, cognitive,
emotional, and social development results in the failure to develop into
normal species members, then those experiences must have been readily
available in human history, or another evolutionary mechanism would have
developed (e.g., internally guided development or elimination of that aspect
of development through selection). If essential experiences are denied,
whether through biological defects or through social environmental mala-
daptation (e.g., wars, famines, severe neglect, and abuse), then individual

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