Conclusions 107cortisol response to stress or perceived attack. Suppressed
hostility can lead to cardiovascular disease.
Among the monoamines, serotonin deficit is most highly
associated with impulsive aggression. However, low sero-
tonin is associated with depression and suicide as well as ag-
gression and homicide, another example of the multiple trait
associations of biological markers. Lack of emotional and be-
havioral control is the likely consequence of serotonin deficit.
Depletion of tryptophan, the precursor of serotonin in the
production chain, increases aggressive responses and angry
and hostile feelings in laboratory experiments. Augmentation
of serotonin, through reuptake inhibitors, can reduce aggres-
sion in aggression-prone persons.
Unlike depression, in which both serotonin and norepi-
nephrine depletions are seen, brain norepinephrine (from
CSF) tends to be positively correlated with aggressive ten-
dencies in monkeys and humans. However, low levels of pe-
ripheral levels of the catecholamines norepinephrine and
epinephrine are also related to aggressiveness. We need to
distinguish between the type of aggression that occurs in
states of high emotional arousal and the cold type of aggres-
sion more characteristic of the psychopath. The latter type
may be reflected in the low levels of peripheral cate-
cholamine reactivity.
Testosterone is associated with aggression based on be-
havioral records, but results using self-report measures of
hostility or aggression are less conclusive. Prisoners with ei-
ther histories of extremely violent crimes or characterized by
aggression in prison show high levels of testosterone. Testos-
terone is increased by victory in competitive contests and
sexual stimulation and decreased by defeat, raising the old
“chicken or egg” problem of causation. The influence of
testosterone during development may be mediated by its in-
fluence on physique in male adolescents where it is associ-
ated with a more muscular mesomorhpic body build. Low
cortisol levels are found in aggressive types and are also in-
fluenced by the outcomes of fights.
Aggression trait is moderately heritable, but its heritability
depends on the form it takes. Assaultive aggression is moder-
ately heritable but verbal aggression is only weakly heritable.
The gene for MAO of the A type has been linked to aggres-
sion in a human family study. Deletion of the MAO-A gene in
mice increases their aggressivity, suggesting that the gene is
involved in the inhibition or regulation of aggression.
CONCLUSIONS
Wilson (1998) described consilienceas a quality of science
that links knowledge across disciplines to create a common
background of explanation. Personality psychology, extend-
ing from social psychology at the higher level to biopsychol-
ogy at the more fundamental level, provides a daunting
challenge to consilience. The introduction to this chapter pre-
sented a model of levels along the biological and social path-
ways leading up to a merger in personality traits.
Such a levels approach suggests a goal of reductionism, a
pejorative term for critics of science and many scientists as
well. The artist is contemptuous of the critic’s attempts to re-
duce his or her art to a textual formula, and the social scien-
tist may resent the presumptious intrusion of the biological
scientist into his or her own complex type of explanation.
Wilson, however, views reductionism as a natural mode of
science:The cutting edge of science is reductionism, the breaking apart
of nature into its natural constituents.... It is the search strategy
employed to find points of entry into otherwise impenetrably
complex systems. Complexity is what interests scientists in the
end, not simplicity. Reductionism is the way to understand it.
The love of complexity without reductionism makes art; the love
of complexity with reductionism makes science. (pp. 58–59)Later, Wilson (1998) admits that reductionism is an over-
simplification that may sometimes be impossible. At each
level of organization the phenomena may require new laws
and principles that cannot be predicted from those at more
general levels. My view is that this is always true for levels
that involve an interaction between biological traits or genes
and experience in the social environment. A learned associa-
tion cannot be reduced to a specificset of neural events, at
least not in the complex brain of a higher organism. It is not
inconceivable, however, that the difference in generalneural
events that make an association more likely in one individual
than another is not only explicable but also essential for a
complete understanding of the event. Consilience is more
possible at the borders of two levels, and this is where the
breakthroughs are most likely to take place. As Wilson puts
it, “The challenge and the cracking of thin ice are what gives
science its metaphysical excitement” (p. 60).
This chapter was organized around a top-down approach,
starting with four broad classes of personality traits that are em-
pirically identifiable across several systems of trait description:
extraversion/sociability, neuroticism/anxiety, impulsiveness/
conscientousness, and aggression/agreeableness. One way to
bypass the complex social determinants of these traits in
human societies is to look for appropriate animal models and
biological links between behavior in these species and
our own. This approach has identified certain biological mark-
ers for analogous behavioral traits such as the monoamine