186 II. BIOLOGICAL  FOuNDATIONS
ter health in humans is mediated by individ -
ual health behaviors (Altschul et al., 2018).
In summary, the findings reviewed in this 
section suggest that studies of personality in 
animals have revealed weaknesses of some 
proposed evolutionary and mechanistic ex -
planations of human personality. Studies 
of animal personality have also pointed to 
promising alternative explanations. In terms 
of furthering our understanding of person -
ality, then, the need to study personality in 
animals can be likened to the suggestion 
that we study WEIRD (Westernized, edu -
cated, industrialized, rich, and democratic) 
humans (Henrich, Heine, & Norenzayan, 
2010).
Future  Directions
The evidence that personality can be mea -
sured reliably in animals, and that these 
measures are capturing real differences 
among individuals, both within and between 
species, is hard to dispute. Likewise, the 
studies described in this chapter show how 
animal personality research has informed 
our understanding of human personality, 
with regard to its proximate causes and con -
sequences, and its evolutionary bases. The 
research described thus far represents only a 
small part of the extensive literature on ani -
mal personality research conducted by be -
havioral ecologists, comparative psycholo -
gists, and personality psychologists. Further 
progress could be made by research that an -
swers the growing calls for an integration of 
these still-  distinct research strands (Carter, 
Feeney, et al., 2012; Koski, 2011a; Nettle & 
Penke, 2010; Weiss & Adams, 2013).
One set of studies could examine associa -
tions between the domains and facets of the 
five- factor model and behavioral measures 
such as boldness, exploratory behavior, 
and so forth (Réale, et al., 2007), using a 
multitrait–  multimethod framework (Camp -
bell & Fiske, 1959). Although animal re -
searchers have examined how personality 
dimensions assessed by ratings are related to 
specific behavioral observations (e.g., Peder -
son, King, & Landau, 2005) and behavioral 
tests (e.g., Carter, Marshall, Heinsohn, & 
Cowlishaw, 2012a), no comparable human work has been done. Such studies could im -
prove the measures and clarify what con -
structs are being measured by behavioral 
ecologists.
Future research is also needed on a wider 
range of species to fill in what one might 
describe as “phylogenetic gaps.” Next to 
nothing is known about personality in a vast 
array of primate and nonprimate species. 
Carefully selecting which species to study 
and use of phylogenetic analyses (Nunn, 
2011) could lead to new findings.
Finally, there is evidence that human per -
sonality structure may differ in societies 
that more closely resemble those of our early 
human ancestors (Gurven, Von Rueden, 
Massenkoff, Kaplan, & Lero Vie, 2013). 
Likewise, there is evidence that the person -
ality structure of chimpanzees and bonobos 
in captive environments may differ from 
their wild counterparts (Garai, Weiss, Arn -
aud, & Furuichi, 2016; Weiss et al., 2017). 
However, because of differences in how per -
sonality was measured in both sets of stud -
ies, it is too early to conclude whether these 
differences reflect environmental influences 
or evolved differences brought about, for 
example, by self-  domestication (Hare, Wob -
ber, & Wrangham, 2012). Studies using the 
same personality measures in both captive 
and wild environments would help to resolve 
this question.
Conclusions
I hope that this chapter has shown the use -
fulness of studying animal personality, 
whether by behavioral ecologists or com -
parative psychologists, without giving the 
impression that it is the only way to learn 
about personality. Although an integrative 
research agenda would be useful for answer -
ing some questions, the diverse personality 
research ecosystem on display in this volume 
will ensure that other questions are not ig -
nored, and perhaps are answered, too.
ACKNOWLEDGMENT
Sections of this chapter were adapted from Weiss 
(2018), published under the Creative Commons 
Attribution License.