In a hypothesis proposed by Cacioppo and colleagues, the isolation of a member of a social species has detrimental biological effects. In a 2009 review, Cacioppo and Hawkley noted that the health, life, and genetic legacy of members of social species are threatened when they find themselves on the social perimeter.[4] For instance, social isolation decreases lifespan in the fruit fly; promotes obesity and type 2 diabetes in mice;[27] exacerbates infarct size and oedema and decreases post-stroke survival rate following experimentally induced stroke in mice; promotes activation of the sympatho-adrenomedullary response to an acute immobilisation or cold stressor in rats; delays the effects of exercise on adult neurogenesis in rats; decreases open field activity, increases basal cortisol concentrations, and decreases lymphocyte proliferation to mitogens in pigs; increases the 24-hour urinary catecholamine levels and evidence of oxidative stress in the aortic arch of rabbits; and decreases the expression of genes regulating glucocorticoid response in the frontal cortex.
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