One prominent effect of social isolation is – you guessed it – increased anxiety and stress.
Many studies find that removing animals from their cage buddies increases anxiety-like behaviors and cortisol, the primary stress hormone. Human studies also support this, as people with small social circles have higher cortisol levels and other anxiety-related symptoms similar to socially deprived lab animals.
Evolutionarily this effect makes sense – animals that lose group protection must become hyper-vigilant to fend for themselves. And it doesn’t just occur in the wild. One study found that self-described “lonely” people are more vigilant of social threats like rejection or exclusion.
Another important region for social homeostasis is the hippocampus – the brain’s learning and memory center. Successful social circles require you to learn social behaviors – such as selflessness and cooperation – and recognize friends from foes. But your brain stores tremendous amounts of information and must remove unimportant connections. So, like most of your high school Spanish – if you don’t use it, you lose it.