Critics challenge contagious traits study

In a handful of recent papers, outside scientists critiqued certain methods Christakis and Fowler used to reach their striking conclusions.

The critics point out specific ways in which the researchers’ analyses didn’t adequately rule out reasons other than social contagion that could cause a group to share a trait, including living in a common environment or the tendency for people to choose friends who are similar to them. Are you lonely because you “caught’’ it from a friend in your network? Or is that one of the reasons you became friends?

The debate is a classic example of how science works. In a back-and-forth that can drag out for years, researchers argue the merits of studies at scientific meetings, in letters published in journals, and, increasingly, on blogs. But this case is unusual because the ideas were popularized by the media and the scientists even as specialists challenged the foundations of the findings. The publicity has drawn scholars from mathematics and political science into a debate about public health.

“Probably if Christakis and Fowler had not pushed it quite so forcefully and vigorously, probably not so many people would have responded as vigorously,’’ said Cosma Shalizi, an assistant professor of statistics at Carnegie Mellon University who co-authored one critique of the work.

“It was actually kind of depressing. When we started working on it, I thought what was in the original Christakis-Fowler paper wasn’t quite right and there’d be a small technical fix. I was rather surprised,’’ Shalizi said, when his analysis suggested that using their approach, it was generally not possible to distinguish contagion from homophily - birds of a feather flocking together - without making what he called implausible assumptions about how the network is formed.