ous, so researchers will measure changes in children’s
physiology as well. Shy children, for instance, tend to
experience a higher heart rate when they are in new
situations compared to when they are at home. Chil-
dren are also asked to report their perceptions of
their temperament style after around the age of eight.
This is when most children are able to report their
own behaviors and preferences in a reliable manner.
Few studies, however, include self-report measures
because most temperament studies focus on children
in infancy and early childhood.
Biological Factors
Since theorists have argued that temperament
has biological roots, many studies have focused on ge-
netic and neurological correlates of different behav-
ioral styles. Most (if not all) temperament dimensions
appear to be moderately heritable, with shyness show-
ing the highest heritability. That is, the more closely
people are genetically related, the more alike they are
in temperament. Much of the evidence supporting
this conclusion comes from twin studies, which com-
pare the behavioral similarity of monozygotic (identi-
cal) twin pairs to dizygotic (fraternal) twin pairs.
Monozygotic twins inherit identical genotypes be-
cause they develop from the same fertilized egg. In
contrast, dizygotic twins inherit, on average, 50 per-
cent of their segregating genes. If genetic differences
across people are associated with temperament differ-
ences across people, then identical twin similarity
should be twice as high as fraternal twin similarity.
Plomin and others found that this is true for tempera-
ment characteristics such as emotionality, activity
level, sociability, and shyness.
Interestingly, some studies show that identical
twins are more than twice as similar as both fraternal
twins and other pairs of relatives (e.g., parents and
their children and biologically related siblings). How
could this be? David Lykken and his colleagues hy-
pothesized that temperament (and personality) dif-
ferences are associated with genetic effects that do not
run in families. These genetic effects are the result of
complex interactions across loci at the level of the ge-
nome and across behaviors at the level of the develop-
ing person. Only identical twins inherit all the genes
associated with these higher-order interactions, and
so they will be much more similar to each other com-
pared to other pairs of genetically related relatives.
Some social scientists maintain that high identical
twin similarity on temperament measures is due to
monozygotic twin assimilation effects (i.e., parents
treat identical twins more alike compared to fraternal
twins) or to measurement problems (i.e., measures
are not sensitive enough to detect moderate to low
fraternal twin or sibling similarity). Still, few research-
ers would argue that temperament is completely de-
termined by the environment.
During the 1990s, the search for biological corre-
lates of temperament differences expanded to in-
clude investigations of brain activation patterns.
Scientists found brain activation differences between
children who approach new situations (i.e., behavior-
ally uninhibited children) compared to children who
withdraw from novel contexts (i.e., behaviorally in-
hibited children). Even though temperament styles
appear to be linked to genetic, physiological, and
neurological processes, temperament researchers still
consider environmental factors to be very important.Environmental Factors
Thomas and Chess argued that children’s tem-
perament characteristics interact with parenting to
produce children’s positive or negative adjustment.
Their concept of the goodness-of-fit between the par-
ent and the child is similar to the notion of attach-
ment developed by John Bowlby. Attachment is the
dynamic relationship between the child and the care-
giver. Human infants are born vulnerable and need
the security of a consistent, attentive, warm caregiver
in order to feel safe enough to explore the world. Car-
egivers give children verbal and nonverbal clues
about the nature of the environment and provide
them with a secure base to return to when they feel
anxious or threatened.
Mary Ainsworth advanced the attachment litera-
ture by creating a laboratory measure of attachment
called the Strange Situation. During this procedure,
the child and caregiver are separated and reunited
several times in a laboratory playroom. During the
separation episodes, the child is left alone with a
strange adult or in a strange room for short periods.
The level of distress the child exhibits after the care-
giver returns is the index of the strength of the attach-
ment relationship. Securely attached infants will
become upset during separation, but can be easily
consoled by the caregiver when reunited. Insecurely
attached infants will show one of two different reac-
tions to the situation. In one group, the insecurely at-
tached infants showed little distress when left alone
and freely interacted with a stranger. In a second
group, the infants were very distressed when left
alone and could not be comforted by their caregivers
upon their return. Many temperament researchers
have pointed out that the Strange Situation is not
equally strange (or scary) for all children. Very emo-
tional or shy children may react strongly to the novel
context, while very sociable children may show no dis-
tress at all. Some researchers even contend that Ains-
worth’s measure of attachment is really assessingTEMPERAMENT 411