In ontogeny and phylogeny, humans need others to survive and prosper. Animal studies of social isolation are an important complement to human studies because randomization and experimental manipulations of isolation in humans are limited in intensity and duration by the possible damaging effects. In addition, longitudinal studies of social isolation in population-based samples with statistical controls for potential confounding variables (e.g., negativity, depressive symptomatology) have begun to identify potential behavioral, neural, hormonal, cellular, and genetic effects of isolation in humans.21,22
Among the findings in this research are that: (1) perceived social isolation is a more important determinant of deleterious outcomes than is the variation in objective social isolation that is seen in population based studies; and (2) the effects of perceived isolation in these longitudinal studies share much in common with the effects of experimental manipulations of isolation in nonhuman social species: increased tonic sympathetic tonus and HPA activation and decreased inflammatory control and immunity; expression of genes regulating glucocorticoid responses; and glucocorticoid resistance.
Humans predate one another, and humans are capable of deception, betrayal, exploitation, and murder as well as empathy, compassion, loyalty, and prosocial behavior. Given shifting alliances and malleable social hierarchies, the presence of others, even ostensive friends, may constitute a social threat at any moment in time. The objective presence of others, therefore, is not sufficient to ensure the social connections needed for human survival and prosperity. Fortunately, assessments have been developed with which to measure perceived as well as objective isolation. Studies using these assessments indicate that objective social isolation can affect perceptions of isolation,23,24
but show that perceived social isolation is more closely related to the quality than the quantity of social interactions.25
When the effects of each are contrasted, perceived social isolation predicts various outcomes above and beyond what is predicted by objective isolation. Beyond what is predicted by objective isolation, perceived isolation predicts greater vascular resistance,26
elevated blood pressure,27,28
morning rise in cortisol,29
less salubrious sleep,28,30
and sedentary lifestyles.31
Perceived (but not objective) social isolation has also been associated with gene expression—specifically, the under
-expression of genes bearing anti-inflammatory glucocorticoid response elements and over
-expression of genes bearing response elements for pro-inflammatory NK-κB/Rel transcription factors.32
This finding is paralleled by decreases in lymphocyte sensitivity to physiological regulation by the hypothalamic pituitary adrenocortical (HPA) axis in individuals who feel socially isolated.33
Together with evidence of increased activity of the HPA axis,29,34
these results suggest the development of glucocorticoid resistance in individuals who feel chronically isolated.
In a longitudinal study of cognitive functioning, Wilson et al.35
reported that perceived social isolation predicted cognitive decline and risk for Alzheimer’s disease (AD). Importantly, perceived isolation persisted in predicting each of these outcomes even when social network size and frequency of social activity were statistically held constant. Similarly, perceived isolation has been found to predict lifetime change in IQ36
and changes in depressive symptoms37,38
beyond what could be predicted by objective isolation. Experimental manipulation of social isolation39
and imagined future isolation40
result in cognitive and behavioral changes even though objective isolation is not altered in these experimental studies. An experience sampling study, in which participants were beeped randomly nine times per day for seven days, confirmed that the social interactions of individuals who feel isolated, in contrast to connected, were more negative and less satisfying, and such interactions contributed subsequently to more negative moods and interactions.41
Perceived social isolation, known more colloquially as loneliness, was characterized in early scientific investigations as “a chronic distress without redeeming features”.42
Recent research suggests that the social pain of loneliness evolved as a signal that one’s connections to others are weakening and to motivate the repair and maintenance of the connections to others that are needed for our health and well-being and for the survival of our genes.43
Physical pain is an aversive signal that evolved to motivate one to take action to minimize damage to one’s body. Feeling socially disconnected and isolated triggers social pain, an aversive signal that evolved to motivate one to take action that minimizes threats or damage to one’s social body. Research on social rejection, for instance, suggests that social pain co-opted the physical pain to extend its protective function to include those with whom we form connections. In Eisenberger, Lieberman, and Williams,44
participants were excluded from or included in a social situation (i.e., a ball tossing game). Results revealed neural activation localized in a dorsal portion of the anterior cingulate cortex (dACC) that is implicated in the affective component of the physical pain response. Eisenberger and her colleagues suggest that, “Because of the adaptive value of mammalian social bonds, the social attachment system… may have piggybacked onto the physical pain system to promote survival” (p. 291).
Open field behavior in nonhuman animals represents a compromise between predator evasion tactics and the reinstatement of contact with an ingroup.45,46
Evidence from behavioral and fMRI studies suggest that social isolation increases attention to negative social stimuli (e.g., social threats) as well as increased motivation to reconnect. Using a modified emotional Stroop task, individuals who felt isolated, relative to those who felt connected, showed greater Stroop interference, specifically for negative social relative to negative non-social words.47
No differences were found in Stroop interference for positive social relative to positive non-social words. Stroop interference is used to gauge the implicit processing of stimuli, so these results suggest that perceived isolation is associated with a heightened accessibility of negative social
information. Similarly, Yamada and Decety48
investigated the effects of subliminal priming on the detection of painful facial expressions. Using signal detection analyses, they found that although the pain was more easily detected in dislikable than likable faces overall, lonely individuals were more sensitive (d’) to the presence of pain in dislikable faces than were nonlonely individuals.
In an fMRI study, we found that individual differences in perceived isolation predicted the activation of the visual cortex to the presentation of unpleasant social, in contrast to nonsocial, pictures, consistent with the remnants of greater predator evasion and a stronger focus on self-preservation in individuals who feel isolated. To examine this possibility, activation in the temporoparietal junction (TPJ)—a region that has been found previously to be activated in theory of mind tasks and in tasks in which individuals take the perspective of another—was also examined.47
TPJ activation was observed when participants viewed unpleasant pictures of people versus objects and, as would be expected if isolation increases a focus on predator evasion and self-preservation, individual differences in perceived social isolation were inversely related to the amount of activation observed in the TPJ.
We also investigated the extent to which individual differences in perceived isolation predicted differential activation of the ventral striatum to pleasant social versus matched nonsocial images.47
The ventral striatum, a key component of the mesolimbic dopamine system, is rich in dopaminergic neurons and is critical in reward processing and learning.49,50
The ventral striatum is activated by primary rewards such as stimulant drugs,51
abstinence-induced cravings for primary rewards,52
and secondary rewards such as money.53
Evidence that social rewards also activate the ventral striatum has begun to accumulate in studies of romantic love,54
and punitive altruism.57
We found that perceived isolation predicted weaker activation of the ventral striatum to pleasant pictures of people than of equally pleasant pictures of objects. These results raise several interesting questions. As noted above, an experience sampling study indicated that individuals who felt socially isolated regard pleasant interpersonal interactions as less pleasant than do individuals who feel socially connected.41
Does perceived isolation modulate the responsiveness of rudimentary reward systems to social and nonsocial stimuli, or do preexisting differences in dopaminergic response to appetitive social and nonsocial stimuli lead to differences in social isolation? Although animal studies may be particularly informative, the finding that individual differences in perceived social isolation are about 50% heritable increases the plausibility of the latter hypothesis.