TABLE 1
SOURCE: David F. Bjorklund and Jesse M. Bering.
Physical Milestones
The list for physical development (Table 1) in-
cludes a number of familiar milestones for infants,
most related to gaining control over their bodies so
that they are able to move about on their own. The
list of milestones for the preschool years will also be
familiar. It is during this time that children become
toilet trained and learn to use simple tools, such as
forks and spoons. The first permanent teeth erupt
around six years of age. Although children across the
globe are typically weaned by age three or four, they
are not able to eat an adult-style diet until they have
most of their permanent teeth. This means that adults
must specially prepare food for children years after
they have stopped nursing. This is a pattern seen in
no other animal and makes the period of ‘‘childhood’’
unique to the human species.
Physical growth is slow and gradual between the
ages of about six and eleven, when the adolescent
growth spurt begins (sometimes a bit earlier for girls).
The rapid growth at this time, which occurs later for
boys, coupled with the development of secondary sex-
ual characteristics, marks the physical transition to
adulthood. Girls’ first menstrual period (menarche)
usually occurs about two years after the onset of sec-
ondary sexual characteristics, and both boys and girls
have a period of relative infertility, lasting several
years, after they have become sexually mature. Al-
though the pattern of puberty described here is uni-
versal, the average age at which girls reach puberty
has been decreasing over the past two centuries, pri-
marily because of better nutrition and health. There
is also evidence that girls from high-stress, father-
absent homes reach puberty earlier than girls from
low-stress, father-present homes, reflecting the role
that social factors can have on physical development.Cognitive Milestones
Selected cognitive milestones are presented in
Table 2. It is not until around seven or eight months
that infants will search for an object hidden as they
watch, believing, apparently, that the object continues
to exist even though they no longer see it. First words
are usually uttered late during the first year, and chil-
dren’s first two-word sentences are typically spoken
between eighteen and twenty-four months of age.
Language abilities develop rapidly during the third
year of life, so that by age three and a half, most chil-
dren are linguistic geniuses, being able to speak their
native tongue proficiently (and far better than most
adult second-language learners). Children have a dif-
ficult time taking the psychological perspective of
others until about three and a half to four years of
age. Until this time, they often believe that if they
know something (for example, that a cookie has been
moved from a box to a jar), other people should know
it as well, even though others have different knowl-
edge (not knowing the cookie was moved). Under-
standing that people’s behavior is governed by beliefs
and desires, which may be different from one’s own,
has been termed ‘‘theory of mind’’ and is the basis of
all sophisticated human social interaction. ThinkingMILESTONES OF DEVELOPMENT 273