Am copying and pasting the most comprehensive answer that i found...
First, picture a monkey. A monkey dressed like a little pirate, if that helps you. We'll call him Slappy.
Imagine you have Slappy as a pet. Imagine a personality for him. Maybe you and he have little pirate monkey adventures and maybe even join up to fight crime. Think how sad you'd be if Slappy died.
Now, imagine you get four more monkeys. We'll call them Tito, Bubbles, Marcel and ShitTosser. Imagine personalities for each of them now. Maybe one is aggressive, one is affectionate, one is quiet, the other just throws shit all the time. But they're all your personal monkey friends.
Now imagine a hundred monkeys.
Not so easy now, is it? So how many monkeys would you have to own before you couldn't remember their names? At what point, in your mind, do your beloved pets become just a faceless sea of monkey? Even though each one is every bit the monkey Slappy was, there's a certain point where you will no longer really care if one of them dies.
So how many monkeys would it take before you stopped caring?
That's not a rhetorical question. We actually know the number.
You see, monkey experts performed a monkey study a while back, and discovered that the size of the monkey's monkey brain determined the size of the monkey groups the monkeys formed. The bigger the brain, the bigger the little societies they built.
They cut up so many monkey brains, in fact, that they found they could actually take a brain they had never seen before and from it they could accurately predict what size tribes that species of creature formed.
Most monkeys operate in troupes of 50 or so. But somebody slipped them a slightly larger brain and they estimated the ideal group or society for this particular animal was about 150.
Read more directly here: http://www.cracked.com/article_14990_what-monkeysp...