From Sociobiology: The New Synthesis by E.O. Wilson
The availability and quality of food can also move groups along behavioral scales. Well-fed honeybee colonies are very tolerant of intruding workers from nearby hives, letting them penetrate the nest and even take supplies. But when the same colonies are allowed to go without food for several days, they attack every intruder at the nest entrance. In general, primates also become increasingly intolerant of strangers and aggressive toward other group members during times of food shortages.
I tend to doubt it is a coincidence that this unprecedented and uncharacteristic openness to immigration and disregard for national borders is entirely unrelated to the fact that the West is fatter and more overfed than ever before in human history.
With societies, as with individuals, lean is mean, hard, and dangerous. Fat is soft, weak, and defenseless.