"We found there appears to be a genetic tendency to introduce your friends to each other," Christakis said.

There could be good, evolutionary reasons for this. People in the middle of a social network could be privy to useful gossip, such as the location of food or good investment choices.

But they would also be at risk of catching germs from all sides -- in which case the advantage would lie in more cautious social behavior, they wrote in the Proceedings of the National Academy of Sciences.

"It may be that natural selection is acting on not just things like whether or not we can resist the common cold, but also who it is that we are going to come into contact with," Fowler said in a statement.

克里斯塔奇斯说：“我们发现人们在介绍朋友互相认识时有一个基因倾向性。”

这种倾向性可以用合理的、进化方面的原因来解释。处于社交圈中的人们可能会参与一些有用信息的传播，比如哪些地方有吃的，或者好的投资选择等等。

但研究人员在发表于《国家科学院院刊》上的研究报告中称，在这一过程中，人们也会受到来自各方的一些不利影响——由此说来，只有谨慎的社交活动才能给人带来好处。

福勒在一份声明中说：“可能自然选择不仅会影响我们是否能够抵抗普通感冒之类的问题，它对我们接触什么样的人也有影响。”

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