period. Although language development is a process
that psychologists have long debated, there is agree-
ment that a strong biological basis for language acqui-
sition exists. In 1967, Eric Lenneberg first proposed
the notion of a critical period for language acquisi-
tion. He suggested that the period between infancy
and puberty (the beginning of adolescence) was a
critical period for language acquisition. This critical
period was thought to end at puberty because of im-
portant maturational changes in the brain that occur
at this time. Language must be acquired during the
critical period if it is to be acquired at all. Alternative-
ly, if the period from infancy to puberty is viewed as
a sensitive period, rather than a critical period, lan-
guage will be learned most easily during this period.
After the sensitive period, language can be learned,
but with greater difficulty and less efficiency.
What evidence is there supporting this concept of
a critical period for language acquisition? How could
a researcher ever test this idea? Information about
this issue comes from different sources. These sources
include a few unfortunate and extreme cases of child-
hood deprivation—children who were deprived of
typical social experiences and stimulation.
Perhaps the most well-known of these was the
case of Genie, who was described in a series of publi-
cations in the 1970s. Genie was essentially kept in iso-
lation by a maltreating parent, with no exposure to
language and normal social experiences for the time
between toddlerhood and early adolescence. When
she was discovered, she had some understanding of
language but did not speak. After nearly one year of
intensive training and instruction, she had a vocabu-
lary of about 200 words and was speaking in two-word
sentences. Six years later, she had made much prog-
ress, but she was still much less advanced in her lan-
guage than other people her age who had normal
experiences growing up.
Since Genie was able to acquire language follow-
ing the onset of adolescence, the notion that language
learning is impossible after the critical period cannot
be supported. Rather, language learning can occur
after the onset of adolescence but may be incomplete.
The period between infancy and adolescence, there-
fore, may be a sensitive period for language learn-
ing—language can be acquired more easily during
this time—rather than an absolutely critical period.
Infant-Parent Attachment: A Critical/
Sensitive Period for Social Development
Illustrative examples of the concept of a critical/
sensitive period can also be found in the domain of
social development. One particularly interesting ex-
ample is the formation of the infant-parent attach-
ment relationship.
Attachment is the strong emotional ties between
the infant and the caregiver. This reciprocal relation-
ship develops over the first year of the child’s life, and
especially during the second six months of the first
year. During this time, the infant’s social behavior be-
comes increasingly organized around the principal
caregiver.
John Bowlby, a twentieth-century English psychi-
atrist who was strongly influenced by evolutionary
theory, formulated and presented a comprehensive
theory of attachment. In the late 1950s and early
1960s, he first proposed that there is a strong biologi-
cal basis for the development of this relationship. Ac-
cording to Bowlby, the infant-parent attachment
relationship develops because it is important to the
survival of the infant and also provides a secure base
from which the infant can feel safe exploring their en-
vironment.
Bowlby suggested that there was a sensitive peri-
od for the formation of the attachment relationship.
This period is from approximately six months to
twenty-four months of age and coincides with the in-
fant’s increasing tendency to approach familiar care-
givers and to be wary of unfamiliar adults. In
addition, according to Bowlby and his colleague Mary
Ainsworth, the quality of this attachment relationship
is strongly influenced by experiences and repeated
interactions between the infant and the caregiver. In
particular, Ainsworth’s research, that was first pub-
lished in the late 1960s, demonstrated that a secure
attachment relationship is associated with the quality
of caregiving that the infant receives. More specifi-
cally, consistent and responsive caregiving is associat-
ed with the formation of a secure attachment relation-
ship.
If the period from six months to twenty-four
months is viewed as a critical period for the develop-
ment of the attachment relationship, the relationship
must be formed during this specific period in early
development. Alternatively, if this period is viewed as
a sensitive period, the infant-parent attachment rela-
tionship will develop more readily during this period.
After the sensitive period, this first attachment rela-
tionship can develop, but with greater difficulty. As in
the case of language development, information about
whether there is a critical or sensitive period for the
formation of a secure attachment relationship comes
from different sources. These sources include cases of
infants who did not experience consistent caregiving
because they were raised in institutions prior to being
adopted.
The early research documenting such cases was
published in the 1940s. This research consistently re-
ported that children reared in orphanages for the
first years of life subsequently exhibited unusual and102 CRITICAL/SENSITIVE PERIODS