 7. Personality in Animals  177
is addressing questions that Gosling, and no 
doubt others, described as being particularly 
amenable to studies of nonhuman animals.
The Historical Development  
of Animal Personality Research
The scientific study of animal personal -
ity can be traced to Charles Darwin and to 
major figures in psychology, including Ivan 
Pavlov, Robert Yerkes, and Donald Hebb. 
However, until the 1970s, animal person -
ality research by comparative psycholo -
gists was sporadic (Weiss & Gartner, 2016; 
Whitman & Washburn, 2017).
The burst of research into animal person -
ality that followed came largely from within 
behavioral ecology, an offshoot of the study 
of animal behavior in biology. Researchers in 
this area seek to answer questions about the 
proximate (physiological, environmental, 
and developmental) and ultimate (functional 
and evolutionary) causes of animal behavior 
(Tinbergen, 1963). A central tenet of behav -
ioral ecology is that species evolve to behave 
optimally under environmental conditions 
and that behaviors can be mathematically 
modeled (Parker & Maynard Smith, 1990). 
For example, a behavioral ecologist may de -
vise a model to determine the optimal group 
size for a population. The model’s param -
eters might include greater vigilance against 
predators (a benefit) and greater competition 
for resources (a cost). These models can then 
be tested in various populations.
The behavioral ecologists’ approach has 
been successful at explaining all manner of 
behaviors in animals and in humans (Krebs 
& Davies, 1997). However, like humans, 
not all animals within a species behave as 
predicted by models. As described elsewhere 
(e.g., Réale, Reader, Sol, McDougall, & 
Dingemanse, 2007), for a long time, behav -
ioral ecologists’ models relegated variation 
around the optimum behavior (i.e., individ -
ual differences) to the error term. Accord -
ing to Réale and his colleagues, the situation 
changed with the publication of a study on 
stickleback fish by Huntingford (1976). The 
fish in that study displayed individual differ -
ences in behavior that were consistent across 
different stages of their breeding cycle and 
in the presence of different intruders. More -over, fish that displayed more aggression in 
the behavioral tests were more likely to ex -
plore new environments. This discovery of 
consistent individual differences led biolo -
gists to try to understand why animals do 
not always exhibit the “optimal” behavior 
given a set of circumstances (e.g., Wilson, 
Clark, Coleman, & Dearstyne, 1994) and 
resulted in a rapid growth in animal person -
ality research.
Reliability and Validity
As in the study of human personality, there 
needs to be evidence that the measures of 
personality under study are reliable and 
valid. This is true regardless of whether the 
measures are ostensibly objective measures, 
such as behavioral observations, or osten -
sibly subjective measures, such as ratings. 
Establishing the construct validity of mea -
sures in animals is also important, because 
doing so establishes whether a measure as -
sesses the same construct in different spe -
cies. It also enables researchers to determine 
whether species similarities and differences 
are in line with what one would expect given 
the natural history of the species.
Reliability
Animal personality traits are stable over 
time. The stability of animal personality 
traits has been demonstrated in three meta- 
analyses. The first meta-  analysis (Bell, Han -
kison, & Laskowski, 2009) involved 114 
studies of a wide range of species and trait 
measures classified as belonging to one of 
13 categories: migration, mate preference, 
activity, affiliation, parental, courtship, 
antipredator, foraging, exploratory, aggres -
sion, habitat, mating, and “other.” The au -
thors of this meta-  analysis found a mean 
repeatability— a type of intraclass correla -
tion coefficient (Boake, 1989; Nakagawa 
& Schielzeth, 2010) —of .37 across studies. 
The second meta-  analysis was based on esti -
mates of retest reliability from eight studies 
of nonhuman primates, and found a mean 
retest reliability of .58 across personality di -
mensions such as confidence and excitabil -
ity (Freeman & Gosling, 2010). The third 
meta-  analysis, using a variety of stability 