 7. Personality in Animals  183
2015). Distinct extraversion and agreeable -
ness dimensions therefore emerge in species 
that differ with respect to how much infor -
mation males have about the offspring they 
are related to, and so these dimensions prob -
ably emerged long before modern humans.
Explanations based on reciprocal altru -
ism, the tendency to help unrelated individu -
als because if you do, they are more likely 
to help you in the future (Trivers, 1971), are 
beset with similar problems. For one, recip -
rocal altruism is less likely to evolve in spe -
cies in which individuals do not regularly 
encounter one another or species in which 
individuals would be unable to recognize one 
another (Trivers, 1971). Nonetheless, sepa -
rate agreeableness and extraversion dimen -
sions have been found in orangutans (Weiss 
et al., 2006), but not in brown capuchin 
monkeys (Morton, Lee, Buchanan-  Smith, et 
al., 2013) or mountain gorillas (Eckardt et 
al., 2015), the latter two both being social 
primate species. These findings are not con -
sistent with what one would predict if the 
evolutionary bases of interpersonal person -
ality dimensions in humans (e.g., extraver -
sion, agreeableness, or honesty–  humility) 
are evolutionary products of reciprocal al -
truism. An alternative explanation emerged 
from a study of six macaque species, in 
which Adams et al. (2015) found that the 
personality dimensions defined by traits re -
lated to assertiveness, social confidence, and 
aggression differ as a function of the degree 
to which the species were despotic versus 
tolerant (see Thierry, 2000, for definitions).
Evolutionary accounts of openness to ex -
perience and of conscientiousness also do 
not appear to withstand this kind of scru -
tiny. Proposed benefits of openness have 
included greater creativity, and thus being 
more attractive to the opposite sex; the pro -
posed costs have included proneness to un -
usual beliefs and psychotic disorders (Net -
tle, 2006). However, studies of nonhuman 
primates suggest that openness evolved inde -
pendently in several primate lines and pos -
sibly for other reasons. For example, there 
is evidence that openness evolved in species 
that get some of their food from places that 
are hard to reach or when effort and ingenu -
ity give access to additional food resources 
(see, e.g., Adams et al., 2015; Kone čná et al., 
2008, 2012).For conscientiousness, one evolutionary 
explanation that has been advanced is that 
it enables costly signaling (Buss, 2009) or re -
flects a balance between delayed mating and 
longevity (Nettle, 2006). One might there -
fore predict that conscientiousness should 
be widespread in the animal kingdom. Yet 
despite the finding that variation in two 
aspects of conscientiousness appear to be 
widespread in the animal kingdom (Delgado 
& Sulloway, 2017), as was noted in Gosling 
and John’s (1999) review, in animals, per -
sonality dimensions such as conscientious -
ness are rare.
So what selective pressures may have led 
to the evolution of conscientiousness in our 
species and those other animals that appear 
to possess it? Studies of nonhuman primates 
and other animals suggest that species with a 
conscientiousness dimension, such as chim -
panzees (King & Figueredo, 1997), bono -
bos (Weiss et al., 2015), brown capuchin 
monkeys (Morton, Lee, Buchanan-  Smith, 
et al., 2013), and Asian elephants (Selt -
mann, Helle, Adams, Mar, & Lahdenperä, 
2018) tend to have larger brains. Moreover, 
a study of 36 species, including primates 
and nonprimates, that adjusted for phylog -
eny found that greater self-  control (which is 
central to conscientiousness) was associated 
with a larger absolute brain size and a more 
varied diet (MacLean et al., 2014). However, 
the finding that a conscientiousness dimen -
sion is absent in the large-  brained orangutan 
(Weiss et al., 2006) but present in the small- 
brained common marmoset (Iwanicki & 
Lehmann, 2015; Koski et al., 2017; but see 
Inoue-  Murayama, Yokoyama, Yamanashi, 
& Weiss, 2018) suggests that brain size may 
only partly be responsible. For example, the 
finding of conscientiousness in common 
marmosets may mean that the need for help -
ers to rear young (Burkart, Hrdy, & Van 
Schaick, 2009) may also favor the evolution 
of dimensions resembling human conscien -
tiousness. Thus, Altschul et al.’s (2018) find -
ing that conscientiousness was not related 
to mortality in captive chimpanzees should 
also not be surprising.
In contrast to the previous examples, 
evolutionary explanations for neuroticism 
often draw on the animal literature. This 
may be because traits associated with neu -
roticism differ from the more visible aspects 