70 I. THEORETICAL  PERSPECTI vES AND  CONCEPT uAL uNITS
basic tendencies that refer to the abstract un -
derlying potentials of the individual, where -
as attitudes, roles, relationships, and goals 
are characteristic adaptations that reflect 
the interactions between basic tendencies 
and environmental demands accumulated 
over time. According to McCrae and Costa, 
basic tendencies remain stable across the life 
course, whereas characteristic adaptations 
can undergo considerable change. From this 
perspective, then, a statement such as “Paul 
likes to go to parties because he is extravert -
ed” is not circular, as it would be if “extra -
verted” were merely a description of typical 
behavior (Wiggins, 1997). Instead, the con -
cept “extraverted” stands in for biological 
structures and processes that remain to be 
discovered. This view is similar to Allport’s 
(1937) account of traits as neuropsychic 
structures and Eysenck’s view of traits as 
biological mechanisms (Eysenck & Eysenck, 
1985).
The idea that personality traits have a 
biological basis is also fundamental to Gos -
ling’s (2001; Gosling, Kwan, & John, 2003) 
proposal that personality psychology must 
be broadened to include a comparative ap -
proach to study individual differences in 
both human and nonhuman animals (see 
also Weiss, Chapter 7, this volume). Al -
though scientists are understandably reluc -
tant to ascribe personality traits, emotions, 
and cognitions to animals (Kwan, Gosling, 
& John, 2008), evolutionary theory predicts 
cross-  species continuities for not only physi -
cal but also behavioral traits; for example, 
Darwin (1872) argued that emotions exist in 
both human and nonhuman animals. A re -
view of 19 studies of personality factors in 12 
nonhuman species showed substantial cross- 
species continuity (Gosling & John, 1999). 
Chimpanzees and other primates, dogs, 
cats, donkeys, pigs, guppies, and octopuses 
all showed reliable individual differences in 
Extraversion and Neuroticism, and all but 
guppies and octopuses varied in Agreeable -
ness as well, suggesting that these three Big 
Five factors may capture fundamental di -
mensions of individual differences. Further 
evidence suggests that elements of Openness 
(e.g., curiosity and playfulness) are present 
in at least some nonhuman animals. In con -
trast, only humans and our closest relatives, 
chimpanzees, appear to show systematic in -
dividual differences in Conscientiousness. Given the relatively complex social-  cognitive 
functions involved in this dimension (i.e., 
following norms and rules, thinking before 
acting, and controlling impulses), it makes 
sense that Conscientiousness may have ap -
peared rather recently in our evolutionary 
history. The careful application of ethologi -
cal and experimental methodology, and the 
high interobserver reliability in these studies 
make it unlikely that these findings merely 
reflect anthropomorphic projections (cf. 
Gosling, Kwan, et al., 2003; Kwan, Gos -
ling, & John, 2008). Rather, these surpris -
ing cross-  species commonalities suggest that 
personality traits reflect, at least in part, 
biological mechanisms that are shared by 
many mammalian species.
The most recent biologically based theo -
retical account of the Big Five is the broad 
cybernetic theory proposed by DeYoung’s 
(2015; see also DeYoung, Grazioplene, & 
Allen, Chapter 8, this volume). DeYoung 
and his colleagues view personality traits as 
probabilistic descriptions of relatively stable 
patterns of emotion, motivation, cognition, 
and behavior. They draw on theoretical ac -
counts of the psychological functions that 
unify the personality traits in each Big Five 
domain (e.g., Dennisen & Penke, 2008) and 
have identified candidate brain systems that 
contribute to those psychological functions, 
such as the dopamine system associated with 
reward sensitivity for Extraversion.
In summary, researchers subscribe to di -
verse perspectives on the conceptual status 
of the Big Five, ranging from purely descrip -
tive concepts to biologically based causal 
concepts. This diversity might be taken to 
imply that researchers cannot agree about 
the definition of the trait concept and that 
the field is in disarray. It is important to 
recognize, however, that these perspectives 
are not mutually exclusive. For example, al -
though Saucier and Goldberg (1996) caution 
against drawing inferences about genotypes 
from lexical studies, the lexical hypothesis 
does not preclude the possibility that the 
Big Five are embodied in biological struc -
tures and processes. In our view, “What is a 
trait?” is fundamentally an empirical ques -
tion. Research in diverse areas such as be -
havior and molecular genetics, personality 
stability and change, and accuracy and bias 
in self-  reports and interpersonal perception 
will be instrumental in building and refining 