570 vI. SELF‑ AND S OCIAL P ROCESSES
continue to increase across adulthood, and 
then decline in old age, with a sharp drop 
among those over 90 years of age; this pat -
tern holds across gender, country, ethnicity, 
and birth cohort (Orth, Erol, & Luciano, 
2018; Orth & Robins, 2019).
The increasing crystallization and coher -
ence of the self-  concept also leads to devel -
opmental changes in the rank-order stability 
of self-  esteem, that is, the tendency for in -
dividuals with relatively high (or low) self- 
esteem at one point in time to have relatively 
high (or low) self-  esteem at a subsequent 
point in time. Specifically, self-  esteem stabil -
ity tends to be relatively low in childhood, 
becoming increasingly stable across adoles -
cence and adulthood before declining in old 
age (Trzesniewski, Donnellan, & Robins, 
2003), a developmental pattern mirroring 
that found for personality traits such as the 
Big Five (Lucas & Donnellan, 2011; Specht, 
Egloff, & Schmukle, 2011). By adulthood, 
longitudinal studies of self-  esteem show very 
strong trait effects (about 80% of the inter- 
individual variance) and relatively weak 
state effects (about 20% transient variance 
or measurement error) for self-  esteem (Don -
nellan, Kenny, Trzesniewski, Lucas, & Con -
ger, 2012; Kuster & Orth, 2013; Wagner, 
Lüdtke, & Trautwein, 2016). Overall, the 
evidence supports the notion that self-  esteem 
is an enduring characteristic, comparable in 
stability to basic personality traits.
The previous sections have shown that 
the self emerges fairly early in life, is rela -
tively stable over time, and has universal 
aspects. These conclusions suggest that self- 
awareness, self-  concept, self-  esteem, and 
other aspects of the self may be a product of 
our evolutionary history. If so, we might ex -
pect to see early forms of self in nonhuman 
animals with a shared phylogenetic history.
Is the  Self Distinctive to  Humans?
If omniscient beings from another planet 
were to study the inhabitants of the earth, 
what would stand out about the human spe -
cies? Our use of tools? Our leisure time and 
range of creative endeavors? Our complex 
social organization and interactions? Cer -
tainly, all of these. But what would be most 
salient about the human species would be 
our inner world—the richness of our mental life. Tapping into the inner life of other spe -
cies is unlikely to reveal the same rich stream 
of thoughts, feelings, and intentions.
The capacity for self-  awareness and self- 
representations seems to be a universal 
characteristic of humans. In all human cul -
tures, people have an awareness of their own 
thoughts and feelings, and have relatively 
stable mental representations of themselves. 
The universality of these basic aspects of the 
self is a striking and highly significant fact 
that is often overlooked in light of the sub -
stantial individual and cross-  cultural vari -
ability in the way the self is manifested.
Whenever universal (i.e., species-  typical) 
characteristics are found, scientists generally 
proceed to comparative, cross-  species stud -
ies. The self may be part of human nature, 
but is it unique to humans? Comparative 
psychologists have discovered that a num -
ber of other species show advanced forms of 
self- awareness, based on the same three in -
dicators of self-  awareness studied in young 
children: linguistic, cognitive-  behavioral, 
and emotional markers.
Linguistic  Markers
Language-  trained great apes (chimps, goril -
las, orangutans) show both linguistic mark -
ers of self-  awareness. Koko the gorilla, for 
example, displayed the capacity for reflex -
ive self-  referencing and narrative language 
use when she signed “me love happy Koko 
there” after being shown a picture of her -
self at a birthday party (Patterson & Linden, 
1981, p. 86). As Koko’s statement suggests, 
gorillas might also have personal memo -
ries that serve as the basis for some forms 
of linguistic self-  referencing. Monkeys and 
nonprimates do not show any of these lin -
guistic markers.
Cognitive‑Behavioral  Markers
In a series of seminal studies, Gallup (1970) 
showed that chimpanzees have the capacity 
to recognize themselves in a mirror, leading 
him to conclude that “man may not be evo -
lution’s only experiment in self-  awareness” 
(Gallup, 1977, p. 333). In subsequent studies, 
dozens of animal species have been subject -
ed to the mirror test, but only chimpanzees 
(Gallup, 1970), orangutans (Suarez & Gal -