Comment by coriolanus

Perhaps it's popular because it's not true. But it should be true, and everyone wishes it was true.

The evolutionary basis of xenophobia is well-understood. A quick search turns up, for example:

http://www.independent.co.uk/life-style/health-and-families/...

Babies of about the same age as the ones shown in Obama's tweet already display instinctive xenophobia:

http://www.smithsonianmag.com/science-nature/are-babies-bigo...