The question's not why it's bad, but why it's getting worse

Confusing correlation with causality (or insisting that the former implies the latter) is ubiquitous in public debate on pretty much any given topic. Sometimes this particular error in reasoning seems to be brought about by another type of fundamental mistake, lurking at a deeper level. I'm talking about confusing stock with flow, level with change.

Two examples.

First: we often hear that the recent growth in obesity among low-income Americans is caused by the fact that rich people have better (meaning healthier) food options than poor people. It is definitely true that the poor have worse food options than the rich do. As an explanation, it is also completely irrelevant. What we're trying to explain is the change in obesity among the poor, not the difference in levels of obesity between the rich and the poor. In other words, the question is not why poor people are more likely to be obese than rich people, but why poor people today are much more likely to be obese than poor people were in, say, the 1930s. In order for the availability of healthy food to be the driving force behind that change, it would have to be the case that the contemporary poor eat worse than their counterparts from the past did. And that is patently untrue: as unhealthy as poor people's food options are today, they are much healthier than they were fifty years ago. So no, this can't be it.

Second: homelessness. How often do we hear it's mostly caused by drug addiction and/or mental health problems? And yet, recently, homelessness has been growing while rates of mental health problems have been stagnant, and numbers of individuals with severe addiction problems declining. So again: no, this is definitely not it.