This is Part 2 in a series assessing the claim made by the Wall Street Journal that “sharing a bed is good for your health.” In Part 1, I invited you to think about the questions you should ask yourself when you read these kinds of claims. Here I’ll spell out a few of those questions that could be posed by discerning readers. (Bonus points if journalists begin to ask these kinds of questions more often.)

Let’s start with the headline, “Sharing a bed is good for your health.” At the heart of good research methodology is the question, “Compared to what?” That’s the first question you should ask.

Presumably, the comparison is with not sharing a bed. But are we comparing couples who share a bed with singles who do not? Or couples who share a bed with couples who do not share a bed? If your claims about the health benefits of sharing a bed are based on either of those kinds of studies (comparing singles to couples, or comparing couples who do or do not share a bed), then you actually do not have good scientific evidence for your claim. That’s not for this post, though. In a later post, I will explain why those kinds of studies are not good enough.

“Sharing a bed” seems to suggest that you just need to share the bed and not necessarily have sex in it. When I look at the actual studies, I’ll see where – if at all – sex fits into the equation. That’s the second question.

The claim is that sharing a bed is good for your health. In what way? That’s question #3. What measure of health are we talking about here? Is it your overall sense of how healthy you are? A more objective set of measures, such as cardiovascular health, a more exhaustive assessment of symptoms or diseases, or something else?

Now let’s look at the tease to the longer article: “Men vs. women, couples vs. singles: New studies find benefits in sharing a bed.” That sure makes it sound like couples are benefiting and the poor hapless singles are not. Go ahead and read just one paragraph – the first one – and you will find that claim and more:

“Couples may get health benefits simply from sleeping in the same bed, a burgeoning field of study is showing. In fact, some scientists believe that sleeping with a partner may be a major reason why people with close relationships tend to be in better health and live longer.”

Note how that last sentence just assumes that “people with close relationships tend to be in better health and live longer.” Such claims are often read as suggesting that if you enter a close relationship, you will be in better health and live longer, too. Since the topic is sleeping in the same bed, the “close relationships” under discussion are not with your best friend or the aunt you have been so close to all your life.

I debunked claims about getting married and getting healthier, and getting married and living longer, in Singled Out.

The specific new claim in the article is that there are “benefits in sharing a bed.” The fourth and most important question is, “How do you know?” What kinds of studies were done to document that claim and were they sufficient to do so?

Claims such as “sharing a bed is good for your health” are causal claims. If you share a bed, they proclaim, you will have better health as a result. Is that what the studies really do show?

The question is not about personality. It may be true, for example, that certain personality types like sharing beds more than other types, and that those personality types are also healthier than other types. Showing only that would not be enough to demonstrate that sharing beds makes people healthier. Maybe the key personality types are healthier for other reasons, and the bed-sharing part is totally coincidental.

The key to causal claims is random assignment. Let’s say your question is about couples and you want to know whether couples would be healthier if they shared a bed than if they slept separately. What you would need to do is randomly assign some couples to share a bed and others to sleep separately. (If they got to pick, then personality differences and other such factors could muck things up. With random assignment, personality differences tend to even out across the two groups, especially if you have recruited enough different couples to participate in your research.) Do those assigned to sleep together end up healthier than those assigned to sleep apart?

Note, though, that even that study would not answer the question most relevant to single people who are not sharing a bed: If they did start sharing a bed, would they become healthier? That’s what seemed to be implied by the tease about “Singles vs. couples: New studies find benefits in sharing a bed.”

Bedroom photo available from Shutterstock.