Saturday December 17, 2005
Christmas is a time for pedantic family disputes, and newspaper stories about how alcohol and chocolate are good for you. Imagine, if you can, how Christmassy life would be if we could combine these two elements.
This week, the Daily Mail reported on a scientific study which reported an observation from survey data: that people who drink alcohol in moderation have lower levels of obesity than people who drink heavily. No surprise there. But the same study also found, more interestingly, that people who drink moderately have lower levels of obesity than teetotallers, people who don’t drink at all.
“Scientists said” they began, as is traditional: “drinking a few times a week can cut the risk of obesity by 27% compared to teetotallersâ€¦ Researchers are unsure quite why small, regular amounts of alcohol help to keep drinkers weight downâ€¦” And so on.
Now look. I freely confess, I am possibly one of the pickiest people on the planet. But read those sentences again: this Daily Mail article is suggesting that there is a causal relationship at play here, between moderate drinking and lower rates of obesity.
And that’s not quite what the research showed. It looked at survey data, and found that people who said they were teetotallers had higher rates of obesity than people who said they were moderate drinkers. That’s what it found. It’s perfectly possible, and you might even say, likely, that there is a third, unrelated factor at play here, one that causes people both to be teetotal and have higher rates of obesity.
Let’s be honest here: teetotallers are abnormal. They’re different. They’re not like everyone else. I’m sure they have their reasons for not drinking. And in fact, those reasons might even be our missing link, the factor that causes you to be both a teetotaller and obese: they might be moral, or cultural, perhaps people from ethnic groups who drink less are more likely to be obese; perhaps people who deny themselves the indulgence of alcohol are more likely to indulge in chocolate and chips; perhaps pre-existing ill health will force you to give up alcohol, and make you more likely to be obese, and that’s what’s skewing the figures.
Perhaps these teetotallers are recovering alcoholics: because among the people I know, they’re the ones who are most likely to be absolute teetotallers, and they’re also more likely to be fat, from all those years of heavy alcohol abuse.
And you know what people are like: you can’t trust them as far as you can throw them, especially when you ask them about vice. Perhaps, in the survey, some of the people who said they were teetotallers were just lying about how much they drank, perhaps because they were, in fact, big drinkers, and they were ashamed of that, or in denial about it. Perhaps, even, and I don’t think this is too much of a long shot, people who lie to themselves, and others, are more likely to be obese.
That’s why we do scientific experiments, where we can control as many of the variables as possible. This study is a useful bit of preliminary survey data, but let’s say we care enough about the question that we want to discount these extra possible causal factors we’ve listed above: we’d have to take a group of people, and randomly assign them to a moderate-alcohol or no-alcohol group, and follow up what happened to their weight later. We might see a weight change over time – another hint at a causal relationship – and we’d know that ex-alcoholics, and big chocolate fans, were equally represented in each group.
But it’s also why designing scientific experiments can be difficult sometimes, or even impossible. I can’t take a group of people, with strong religious or cultural reasons for not drinking, and persuade them to be randomised into a group where they would have to drink alcohol.
Outside of the black and white, mechanistic, simple cause and effect universe of the press, finding an association between two things is not enough to say that there is a causal relationship between them. Black people might commit more crime, but it’s very unlikely that this is because the colour of somebody’s skin causes criminal behaviour: in just the same way that drinking moderately doesn’t necessarily make you trim.