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when a scientific study ends by stating that there’s uncertainty about whether a correlation proves a cause, it doesn’t mean that correlations are meaningless in every circumstance. Everyone knows that creating false correlations between two unrelated elements is easy. But it can be that a correlation is so powerful and reliable that it may actually point to that rare thing in the social sciences, a demonstrable causal relation. As a wise man once said, “Correlation is not causation, but it sure is a hint.” When you can separate out a truly robust correlation between two elements in our social life, it’s a big deal. What makes a correlation causal? Well, it should be robust, showing up all over the place, across many states and nations; it should exclude some other correlation that might be causing the same thing; and, ideally, there ought to be some kind of proposed mechanism that would explain why one element affects the other. There’s a strong correlation between vaccines and less childhood disease, for instance, and a simple biological mechanism of induced immunity to explain it. The correlation between gun possession and gun violence—or, alternately, between gun control and stopping gun violence—is one of the most robust that you can find.

Feelings of aggression after playing video games are more likely to be linked to gameplay mechanics rather than violent content, a study suggests.

Who decides what constitutes “proof”? Can “proof” come only from experiments? If so, we’re in trouble, because we can never do an experiment in humans for any exposure that is suspected to be harmful.

The problem is that as new data come along, what was once proven can become unproven again by new expert panels who now consider this new evidence and come to a different conclusion. This leads to the perhaps uncomfortable definition of “proof” as “a consensus of experts.”

The point of emphasis is that there is no hard formula on how to consider the evidence; it is a group decision by a panel of experts (the scientific jury) in which sometimes the epidemiological evidence is so strong that it doesn’t matter whether mice get cancer or not. And sometimes the epidemiology is insufficient to render a verdict on causation and so the toxicology becomes important in making the decision.

Why is it easy for some people to learn to read, and difficult for others? It’s a tough question with a long history. We know that it’s not just about raw intelligence, nor is it wholly about repetition and dogged persistence. We also know that there are some conditions that, effort aside, can hold a child back. Socioeconomic status, for instance, has been reliably linked to reading achievement. And, regardless of background, children with lower general verbal ability and those who have difficulty with phonetic processing seem to struggle. But what underlies those differences? How do we learn to translate abstract symbols into meaningful sounds in the first place, and why are some children better at it than others?

Philosophy is important for more than just a while, and has serious, practical uses for all of society. There are countless examples of philosophy of mind theories’ relevance to neuroscientists, or cases where political philosophers have shaped politicians.

The 18th century philosopher David Hume’s argument that we don’t have a reasonable understanding of causation at all, but only presume cause and effect when two things have been observed as conjoined in the past, is notoriously difficult to refute. The problem underlies much of physics and is hardly insignificant.

A team of researchers walked every street in 228 census tracts around Los Angeles and New Orleans and recorded every outdoor ad they saw. Another group surveyed 2,881 residents of the same census tracts by telephone, paying them to report their height, weight and other information. After analyzing this hard-won data, the authors conclude: “For every 10 percent increase in food advertisements, the odds of being obese increased by 5 percent.” That is, areas with more outdoor food ads have a higher proportion of obese people than ones with fewer ads.

The problem is that their policy recommendations rest on a crucial but unjustified assumption: that any link between obesity and advertising occurs because more advertising causes higher rates of obesity. But the study at hand showed only an association: people living in areas with more food ads were more likely to be obese than people living in areas with fewer food ads. To be fair, the researchers correctly note that additional steps would be needed to prove that food ads cause obesity. But until those steps are taken, talk of restricting ads is premature. In fact, it is easy to imagine how the causation could run the opposite way (something the article did not mention): If food vendors believe obese people are more likely than non-obese people to buy their products, they will place more ads in areas where obese people already live. Suppose we counted ads for fitness-oriented products like bicycles and bottled water, and found more of those ads in places with less obesity. Would it then be wise anti-obesity policy to subsidize such ads? Or would the smarter conclusion be that the fitness companies suspect that the obese are less likely than the fit to buy their products?

When we seek to base policy on evidence, we must remember that not all “evidence” is created equal. Taken at face value, the study on ads and obesity provides some indication that the two are linked, but no evidence that food ads cause obesity. The fact that the causal conclusion may coincide with a moral belief — that it is wrong to tempt people who overeat by showing them ads for food — does not make it valid.

Does bad behavior in movies or other media lead people to behave badly? There’s plenty of research on the link between onscreen media and risky behaviors like unprotected sex, binge drinking, fast driving and even violence. One large meta-analysis of such studies concluded that exposure to risk-glorifying media is associated with risky behaviors by people who consume that media. But causality issues plague most studies in this area: People who engage in risky behaviors may prefer to consume risk-glorifying media. These studies also tend to measure attitudes in controlled lab settings rather than in real life.