Are you a social butterfly, or do you prefer being at the edge of a group of friends? Either way, your genes¹ and evolution may play a major role, US researchers reported .

While it may come as no surprise that genes may help explain why some people have many friends and others have few, the researchers said, their findings go just a little farther than that.

"Some of the things we find are frankly² bizarre," said Nicholas Christakis of Harvard University in Massachusetts, who helped conduct the study.

"We find that how interconnected your friends are depends on your genes. Some people have four friends who know each other and some people have four friends who don't know each other. Whether Dick and Harry³ know each other depends on Tom's genes," Christakis said in a telephone interview.

Christakis and colleague James Fowler of the University of California San Diego are best known for their studies that show obesity⁴, smoking and happiness spread in networks.

For this study, they and Christopher Dawes of UCSD used national data that compared more than 1,000 identical and fraternal twins. Because twins share an environment, these studies are good for showing the impact that genes have on various things, because identical twins share all their genes while fraternal twins share just half.

"We found there appears to be a genetic⁵ tendency to introduce your friends to each other," Christakis said.

There could be good, evolutionary⁶ reasons for this. People in the middle of a social network could be privy⁷ to useful gossip, such as the location of food or good investment choices.

But they would also be at risk of catching⁸ germs from all sides -- in which case the advantage would lie in more cautious social behavior, they wrote in the Proceedings⁹ of the National Academy of Sciences.

"It may be that natural selection is acting¹⁰ 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.

你是爱好交际还是喜欢独来独往？美国研究人员报告称，不管你属于哪一类，你的基因及进化过程可能在这一问题上起着重要的作用。

研究人员称，用基因解释为什么有些人朋友众多而有些人却寡朋少友其实并不新鲜，不过他们的研究更深入一些。

来自马萨诸塞州哈佛大学的尼古拉斯•克里斯塔奇斯协助开展该研究，他说：“我们发现的一些现象的确很奇怪。”

克里斯塔奇斯在一次电话采访中称：“我们发现一个人朋友之间的亲密程度取决于这个人的基因。有的人有四个朋友而且互相认识；而有的人有四个朋友，但他们却素不相识。也就是说，迪克和哈利是否互相认识是取决于汤姆的基因。”

克里斯塔奇斯和他的同事、来自加利福尼亚大学圣地亚哥分校的詹姆斯•福勒此前以研究肥胖、吸烟以及快乐会在朋友之间传播而著名。

在这项研究中，他们和加利福尼亚大学圣地亚哥分校的克里斯托弗•道维斯使用了一项全国性数据，这项数据对1000多对同卵及异卵双胞胎的基因进行了对比。由于双胞胎生活在同一个环境，而且同卵双胞胎的基因完全相同，而异卵双胞胎有一半的基因相同，所以这些研究有助于发现基因对人各个方面的影响。

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。”