182 II. BIOLOGICAL  FOuNDATIONS
addition, the species that one compares can 
either have faced similar challenges in their 
evolutionary history (e.g., both species live 
in large groups) or different challenges (e.g., 
one species lives in an environment in which 
there are many predators, and the other spe -
cies lives in an environment in which preda -
tors are absent).
There are two comparisons that allow for 
strong tests about the evolution of person -
ality. The first is the comparison between 
closely related species that faced different 
challenges. Here, if the species differ in, for 
example, the personality dimensions that 
they possess, the parsimonious explanation 
is that the personality differences evolved re -
cently in response to the challenges. On the 
other hand, if these species possess the same 
set of personality dimensions, the parsimo -
nious explanation is that these personality 
dimensions were present in the common an -
cestor of both species.
The second is the comparison between 
distantly related species that faced similar 
challenges. Here, if the species are similar 
to one another, for example, if both possess 
the same set of personality dimensions, the 
most parsimonious explanation is that these 
personality dimensions are an evolutionary 
product of those challenges. This method 
has been used to test hypotheses about per -
sonality structure, associations between 
personality traits and other variables, and 
whether certain features of species led to dif -
ferences in mean levels of traits.
Structure
To test hypotheses concerning the evolu -
tion of personality structure, researchers 
have been gathering data on one of several 
animal personality questionnaires and/or 
on naturally occurring behaviors (Freeman 
& Gosling, 2010). These researchers then 
use factor analysis or principal components 
analysis to identify personality dimensions 
in the species. The dimensions identified are 
typically a combination of (1) those that are 
human-like (e.g., orangutan neuroticism; 
Weiss, King, & Perkins, 2006), (2) those 
that resemble human personality facets (e.g., 
rhesus macaque anxiety; Weiss et al., 2011), 
(3) those that combine two or more human 
dimensions (e.g., brown capuchin monkey sociability; Morton, Lee, Buchanan-  Smith, 
et al., 2013), (4) those found in multiple pri -
mate and primate species, but not in humans 
(e.g., dominance; Freeman & Gosling, 2010; 
Gosling & John, 1999), and (5) those specif -
ic to a species or taxonomic group (e.g., “op -
portunistic” in Assamese macaques; Adams 
et al., 2015).
The way in which animal studies help 
make sense of personality can be illustrated 
by describing how they inform hypotheses 
about the origins of the five-  factor model. To 
begin, consider extraversion and agreeable -
ness. Compared to other primates, humans 
are intensely social and altruistic (Kurzban, 
Burton-  Chellew, & West, 2015). It is there -
fore not surprising that some researchers 
(e.g., Ashton & Lee, 2007; Nettle, 2006) hy -
pothesize that the evolutionary bases of ex -
traversion and agreeableness can be traced 
to those features of humans. However, per -
sonality variation in social traits has been 
identified in species ranging from the highly 
social chimpanzee (King & Figueredo, 1997) 
to the semisolitary orangutans (Weiss, et al., 
2006), and to the solitary East Pacific red 
octopus (Mather & Anderson, 1993). These 
findings are contrary to the predictions that 
would be derived from theories based on 
human sociality and altruism.
There are also problems with tying social 
traits or their structure to kin selection,  the 
tendency of individuals to help related indi -
viduals at a cost (Hamilton, 1964). For one, 
even across primate species, we see great 
variety in social and mating systems (Crook 
& Gartlan, 1966), level of paternal care 
(Fernandez-  Duque, Valeggia, & Mendoza, 
2009), and other ecological characteristics 
(Galán-Acedo, Arroyo-  Rodríguez, Andre -
sen, & Arasa-  Gisbert, 2019). Still, factor- 
analytic studies of personality in primates 
(Gold & Maple, 1994; King & Figueredo, 
1997; Kone čná et al., 2008) have demon -
strated that distinct dimensions of extraver -
sion and agreeableness exist in chimpanzees 
(a species with a promiscuous mating sys -
tem), as well as in Western lowland gorillas 
and in Hanuman langurs (both species in 
which one male monopolizes access to mul -
tiple females). Mountain gorillas, curiously, 
appear to possess a single dimension, name -
ly, sociability, combining traits related to ex -
traversion and agreeableness (Eckardt et al., 