i read a story when i was a kid ... it went something like this

we all draw a circle around ourselves and the people we want to care about. that is our group. different people draw different circles, some draw it very small and only care about themselves, others care for their family, and other draw an even bigger circle

what i'm wondering is why people draw circles of a specific size. i.e. what causes it that people care about the 300 people in their environment, but not about the other 10 billion people on earth?