Despite a general consensus that oxytocin
(OT) has prosocial effects, there is no clear
agreement on how these effects are achieved.
Human research on OT is reviewed under three
broad research initiatives: attachment and
trust, social memory, and fear reduction. As an
organizing perspective for scholars' current
knowledge, a tentative model of the causes and
effects of alterations in OT level is proposed.
The model must remain provisional until
conceptual and methodological problems are
addressed that arise from a failure to
distinguish between traits and states, differing
research paradigms used in relation to OT as an
independent versus dependent variable, and the
possibility that OT effects depend on the
initial emotional state of the individual.
Social and personality psychologists have
important roles to play in developing more
rigorous and creative research designs.
OT: A Primer and Some Caveats
OT is a peptide hormone composed of nine
amino acids. It is highly conserved across
species in terms of structure and function,
although there is interspecies variability in
the specific behaviors that it controls. OT has
both peripheral and central effects.
Peripherally, OT regulates uterine contractions
during labor and milk ejection during lactation.
It is synthesized in magnocellular neurons of
the paraventricular (PVN) and supraoptic (SON)
nuclei of the hypothalamus, which project to the
posterior pituitary where OT is released into
blood circulation. Centrally, OT acts as a
neuromodulator: Released from all parts of the
neuronal membrane, OT diffuses widely in
extracellular fluid, affecting many regions of
the brain. It is synthesized in the
parvocellular neurons of the hypothalamic PVN,
which projects to limbic sites (hippocampus,
amygdala, striatum, hypothalamus, nucleus
accumbens) and to the brain stem. Central OT
effects include maternal and sexual behavior,
pair bonding, and social recognition.
In some species a small quantity of
peripherally administered OT may cross the
blood&endash;brain barrier or influence behavior
via afferent feedback to the CNS. Studies of
plasma OT also vary in their use of basal versus
reactive measures.
Bear in mind also that although both sexes
have OT receptors, OT is of special relevance to
females because OT synthesis and OT receptors
are upregulated by estrogen. Indeed, McCarthy,
McDonald, Brooks, and Goldman (1996) observe
that the OT receptor is "one of the most
strongly estrogen-regulated systems in the
brain. . . . Estrogen-induced increase in OT
receptor binding is integral to its
behavior-modifying effects".
OT's sister nonapeptide arginine vasopressin
(AVP), which is very similar in structure,
appears to play a more important role in males,
although OT has been shown to affect some male
behaviors including partner preferences, sexual
behavior, and social recognition. Both sexes
have receptors for both neuropeptides, and to
complicate matters further, the structural
similarity of AVP and OT means that they may be
capable of binding to each other's
receptors.
The effects of the same peptide can also
vary dramatically in males and females. For
example, in men intranasally administered AVP
stimulates agonistic facial expressions and
decreased perception of friendliness in response
to images of same-sex strangers. In women
administration of the same peptide results in
affiliative facial expressions and increased
perception of friendliness. Studies that assay
plasma OT as a dependent variable are most often
performed on women, although some studies also
include men. However, nearly all studies where
OT is centrally administered as an independent
variable use male participants only because of
the small possibility of OT entering the
bloodstream where it might cause uterine
contractions. Thus, there is a potential
confound among participant sex, study design
(whether OT is an independent or dependent
variable), and OT system (central vs.
peripheral).
Another note of caution is warranted. Much
of our basic knowledge about OT has come from
research on rodents, and even here there are
important differences between taxa.
Extrapolations to humans must be made with
caution because there is considerable variation
in receptor distribution across mammalian
species. In humans, we do not yet know the
extent to which "hard-wired" responses, such as
maternal behavior, pair bonding, and
affiliation, have been superseded by learning
and cultural transmission afforded by increased
cortical size. There is general consensus that
OT has positive effects on human social
behavior, but there are at least three implicit
proposals about the mediators of these prosocial
effects.
OT: Attachment and Trust
The emotional bond between caregiver and
offspring, and between adult partners, lies at
the heart of the psychology of relationships. A
secure attachment in infancy is important for
normal psychological development and provides a
base from which the infant explores the world
beyond. The infant's internal working model of
attachment has implications for the nature and
quality of later adult relationships. The
continuities and similarities between these two
types of relationship have often been noted by
developmental and social psychologists. It has
been proposed not only that successful adult
pair bonding depends on the early
child&endash;parent relationship but also that
the two share a common psychological mechanism.
Although adult relationships incorporate
sophisticated cognitive, social, and cultural
components, they may share a basic emotional
infrastructure with our earliest experience of
attachment. Because mother&endash;infant
attachment is ubiquitous in mammals, the
possibility of a biological basis attracted
research interest.
Early work on OT focused on its role
supporting maternal behaviors toward offspring
in rodents. In pregnancy, triggered by rising
estrogen levels, OT receptors are upregulated in
the uterus and the brain. Vagino-cervical
stimulation during parturition activates OT
neurons in the hypothalamus, stimulating OT
release in many brain areas including the
preoptic area, ventral tagmental area, and
olfactory bulb. These pathways are responsible
for coordinating a range of maternal behaviors
including nest building, pup retrieval, licking,
crouching, and maternal aggression. In 1979,
Pedersen and Prange first demonstrated that
intracerebroventricular (icv) infusion of OT can
induce maternal responses in estrogen-primed
virgin rats. Reciprocally, the onset of maternal
behavior can be inhibited by OT antagonists,
lesions of OT cells, and antibodies to OT.
Recent studies using genetic knockout of OT
receptors have confirmed significant deficits in
mothering. Even in rodents, the role of OT
however is confined to the initiation not the
maintenance of maternal behavior (Kendrick,
2000). In humans, there is general consensus
that prenatal and postpartum OT both enhances
the formation of close bonds with the infant and
reduces maternal stress reactivity.
