184 II. BIOLOGICAL  FOuNDATIONS
of human personality, because dimensions 
such as neuroticism are found in many spe -
cies (Gosling & John, 1999), or because it 
is difficult to come up with a way in which 
high neuroticism is “beneficial” to humans. 
For example, Nettle (2006) hypothesized 
that costs related to higher neuroticism, in -
cluding poorer health and being at risk of 
psychological disorders, are offset by in -
creased vigilance. Focusing on the African 
apes, this explanation is consistent with neu -
roticism being present in chimpanzees (King 
& Figueredo, 1997), but not bonobos (Weiss 
et al., 2015) or mountain gorillas (Eckardt 
et al., 2015), as the latter two are believed 
to have evolved in predictable, resource- 
rich environments. Studies of humans have 
found further support for this hypothesis in 
that a part of neuroticism related to worry 
and feelings of vulnerability is related to bet -
ter physical health outcomes, including lon -
gevity (Gale et al., 2017; Weiss et al., 2019), 
and that these relationships appear to exist 
at the level of the genome (Hill et al., 2019). 
On the other hand, this explanation is not 
supported by the finding that neuroticism is 
present in western lowland gorillas (Gold & 
Maple, 1994), which also live in a predict -
able, resource-  rich environment. It thus ap -
pears that the proposed benefits of neuroti -
cism only partly outweigh its costs, and that 
further study is needed.
Development
In addition to answering questions relating 
to the maintenance of variation and/or the 
evolution of personality dimensions, com -
parative research has enabled researchers 
to test whether biological factors (McCrae 
& Costa, 2003) or investing in sociocul -
tural roles (Roberts & Jackson, 2009) are 
the drivers of personality development. The 
logic here is that the roles that are hypoth -
esized to influence personality development, 
such as beginning full-time employment, 
are absent or very different in other spe -
cies. In one of these studies, King, Weiss, 
and Sisco (2008) compared chimpanzees to 
humans. In another study, Weiss and King 
(2015) compared chimpanzees to orang -
utans. The authors of these studies took 
into account and adjusted for the different 
rates of development between the great ape species and the humans. They found that 
the direction and magnitude of personal -
ity age differences were mostly comparable 
across species, which appears inconsistent 
with a social roles explanation of personal -
ity development (King et al., 2008; Weiss & 
King, 2015). Moreover, two findings from 
these studies suggest that personality de -
velopment is a product of natural selection. 
First, orangutans declined in agreeable -
ness, suggesting that age-  related increases in 
human and chimpanzee agreeableness may 
be an adaptation for living among unrelated 
conspecifics, which orangutans do not do 
(Weiss & King, 2015). Second, in contrast 
to humans, orangutans, and female chim -
panzees, age-  related declines in extraversion 
leveled off among male chimpanzees (King 
et al., 2008; Weiss & King, 2015), which 
may reflect an adaptation for heightened ag -
gression in chimpanzee males (Wrangham, 
Wilson, & Muller, 2006).
Personality, Happiness, and  Health
Human personality dimensions are associ -
ated with psychological and physical health. 
The study of animal personality provides in -
sight into the mechanisms and evolutionary 
bases that underlie these associations.
In one case, the comparative approach 
has revealed why people who are lower in 
neuroticism and higher in extraversion, and, 
to a lesser degree, higher in the other three 
human personality dimensions, tend to be 
happier (DeNeve & Cooper, 1998; Steel, 
Schmidt, & Shultz, 2008). These findings 
grew from a study by King and Landau 
(2003), who created a four-item question -
naire that allowed raters closely familiar 
with the animals to assess a construct akin 
to subjective well-being in chimpanzees. 
King and Landau’s analyses indicated that 
the four items defined a single factor, the in -
terrater and retest reliabilities were high, and 
the factor was related to higher dominance, 
extraversion, and conscientiousness. Similar 
relationships have been found in other stud -
ies of personality ratings in nonhuman pri -
mates (Robinson et al., 2016; Weiss et al., 
2009, 2011) and felids (Gartner, Powell, & 
Weiss, 2016).
King and Landau’s (2003) findings were 
an initial indication that associations be -