But here's the problem.

但这就有个问题
Here's why it's such a puzzle.

原因就在于
The problem is the existence of cheaters.

总是有骗子
And in economics and sociology

在经济学和社会学范畴中
these are also known as "Free-riders."

骗子被称为搭便车者
And what a cheater or free-rider does is

无论是骗子还是搭便车者
it takes the benefits without paying the cost.

其行为无非是不劳而获
Imagine two genes.

假设有两个基因
Imagine one builds a vampire bat

其中一个基因构成的吸血蝙蝠
that accepts blood from others

能接收来自其他蝙蝠的血液
and shares blood.

同时也分享出去
The other gene accepts blood from others and doesn't share blood.

另外一个基因则只接收不向外分享