In the previous post, I examined the first part of the study on Facebook and mortality rates. Here’s the citation again:
William R. Hobbs, Moira Burke, Nicholas A. Christakis, and James H. Fowler, “Online Social Integration is Associated with Reduced Mortality Risk,” PNAS, Early Edition, published ahead of print on October 31, 2016, doi: 10.1073/pnas.1605554113.
Now it’s time to look at the second part. Having compared the mortality rates of Facebook users and Facebook nonusers, they now investigated to what extent specific Facebook activities were associated with mortality.
To compare the dead and the living, they selected their samples in this manner:
To ensure age and gender covariate balance in our analyses, we compared all deceased individuals on Facebook to a stratified random sample of nondeceased individuals (SI Appendix, Fig. 5) from the full and voter populations described above. There were 179,345 people in our age- and gender-based probability sample of Facebook users born between 1945 and 1989, of whom 17,990 died between January 2012 and December 2013; 89,597 were also present in the California voter record, of whom 11,995 had died between January 2012 and December 2013.
I sense something problematic here, but I’ll have to give it some thought. I understand now (after learning about case control study and revising this paragraph) that the approach is not problematic in itself; it’s statistically valid when applied properly. The question is whether the researchers have applied it properly. I am tabling this for now.
Having identified their sample, they began by comparing two Facebook activities: initiating friendships and accepting friendships. They found a relation between accepting friendships and decreased mortality risk, but no such relation between initiating friendships and the same. Just to be clear about the difference: “(A) Initiated friendship: the subject sent a Facebook friendship request that was then accepted. (B) Accepted friendship: the subject received and accepted a friendship request.” They used a Cox proportional hazard model to estimate the relative risk; they report the details in their appendix.
It appears from their findings that larger numbers of accepted friendships correlate with lower relative mortality; no such correlation exists for initiated friendships. However, it seems a bit arbitrary to examine it in terms of quantity; for instance, they could have examined time lapses between invitation and acceptance, or relation of invited friends to existing friends. I don’t mean that these comparisons would have been better; there’s just no reason to suppose that the number of friend requests or acceptances is particularly important, especially on Facebook.
There’s one way it might be important, though. If you are accepting a lot of friendships, that means others are seeking you out; for whatever reason, they think a Facebook friendship with you is worth a click or two. This may be because they perceive you as healthier. It’s possible that people with debilitating illnesses (especially the ones that correlate with mortality on Facebook) attract fewer friend requests. In that case, it isn’t that the friend-accepting activity makes you live longer; it’s that those accepting friend requests are perceived as vigorous to begin with. (That’s just one of many possible explanations.)
Onward. Next, they compared text-based activity and photo-based activity; they found that “mortality risk declines with increased photos, whereas it actually increases with increased statuses.” Here again I see need to distinguish between correlation and causation. Aren’t healthier people more likely to post photos of themselves and their families (if they’re posting on Facebook to begin with)? Relatively speaking, statuses might appeal more to people who are ill. If you’re unwell, you might use other methods to let people know how you are. You might not want to post pictures.
The authors see photos as indicative of face-to-face interactions; they write, “These results are suggestive that offline social activities—and not online activities—are driving the relationship between overall Facebook activity and decreased mortality risk.” Well, possibly, but it may be that those “offline activities” are already triggered by health or illness.
Next, they consider activities directed at specific individuals: They compared the sending of posts (with tags) and messages with the receiving of photos with tags. Here it’s a little more complicated, but the receiving of photos with tags correlates with lower mortality risk than the sending of posts and messages. Yet this may have to do with one’s existing health status: it could be that those who are healthier tend to receive photos with tags (after all, they made it out to the party or other event), whereas those who are ill may rely on posts and messages.
Finally, they examine how specific Facebook activities relate to rates of mortality for specific causes of death. Then they discuss the results overall. I will take this up next time (probably not today).
My thoughts so far: There may well be an association between certain Facebook activities and mortality, but one’s health status could influence one’s Facebook activity at least as much as vice versa. There’s no reason to believe that the dead people would have prolonged their life if they had engaged in a different Facebook practice.
To shed some light on this, you’d need some information about the subjects’ health. There’s a lot of grey area between “dead” and “alive.”
Update: I learned from a commenter that this is called a case control study. I now wonder whether this is appropriate for a study of Facebook and mortality, given the many possible causes of death. Where case control studies examine the relation between exposure and an “outcome,” I have difficulty seeing death here as an “outcome.” One death may be nothing like another; the causes may be completely unrelated to each other. In addition, while we’re all “at risk” for death, the control subjects might be at negligible risk for these particular deaths; we just don’t know. I’d think the degrees of risk would matter. I welcome any comments on this issue.
Note: I revised this piece substantially after posting it; I later made additional minor edits.