In other words, we need to figure out if this is just a casual relationship or something more -- a causal one.
This is trickier than it sounds, because it's impossible to tell just by looking at sex and wages. The problem is we can't disentangle sex from wages. There just isn't enough information. We need something that shows us that a change in sexual activity changes earnings. We need what economists call an "instrumental variable."
Here's how instrumental variables work. Say we want to figure out if smoking causes health problems. Now, we know that smokers tend to have more health problems, but that doesn't prove that smoking is why they do. It's just a correlation. But what if we look at cigarette taxes instead of smoking itself. All else equal, higher cigarette taxes should mean fewer cigarettes sold -- which, if smoking really is bad for people, should mean better health. Well, lo and behold, higher cigarette taxes are indeed correlated with both less smoking and fewer health problems. And that tells us what's causing what. See, the magic here is there is nothing to disentangle. Cigarette taxes don't help or hurt people's health, except for how they change how much people smoke -- and they do change how much people smoke. So cigarette taxes show us how a change in smoking causes a change in health outcomes. Cigarette taxes are the perfect instrumental variable.
For sex and wages, we need another instrumental variable. Think of something that only changes how much money people make, because it changes how much sex they have? It's not attractiveness. Good-looking people tend to earn more, because they're more productive. And it's not age. Older people tend to earn more, because they're in leadership positions. So what else could it be? Well, Drydakis looks at whether people believe in God or attend at least two religious services a month instead. The idea is that religious people tend to have less sex -- and that having less sex is the only way that being religious affects what they make. But, of course, wanting religiosity to be an instrumental variable doesn't make it one. We need proof. We need to see that there really is a strong relationship between religiosity and sexual activity, but no direct relationship between religiosity and wages. And, believe it or not, that's what Drydakis found when he ran the numbers.
Don't believe it. And here's why you shouldn't, in two sentences. It's reasonable to say, like we did above, that cigarette taxes make us healthier only because they make people smoke less. But it's absurd to say that being religious makes us poorer only because it makes us have less sex. For one, being religious probably tends to make us richer! And, for another, it probably makes us richer for reasons that have nothing to do with sex. Think about it. Church-goers probably have more social capital than others. That means they have a ready community to help them find work or clients when they need to. And going to church probably socializes them to have more of what psychologists call "conscientiousness" -- that is, to be more motivated and hard-working. Religiosity might affect wages in either or both of these ways -- or entirely different ones. But there is zero chance that it only affects wages through how much sex people have.