I talk about the Clean Air Act of 1990 in class and how, like any other regulation, it decreases supply. Any regulation raises costs to producers, so that is why supply is reduced. The price of the good increases and the amount produced falls. But we also hope that regulations have benefits. The first Clean Air Act from the 1970s called for a reduction in lead in gasoline. It turns out that this may have had other benefits by reducing crime. Below is an exerpt from the New York Times article Criminal Element
"The answer, according to Jessica Wolpaw Reyes, an economist at Amherst College, lies in the cleanup of a toxic chemical that affected nearly everyone in the United States for most of the last century. After moving out of an old townhouse in Boston when her first child was born in 2000, Reyes started looking into the effects of lead poisoning. She learned that even low levels of lead can cause brain damage that makes children less intelligent and, in some cases, more impulsive and aggressive. She also discovered that the main source of lead in the air and water had not been paint but rather leaded gasoline — until it was phased out in the 1970s and ’80s by the Clean Air Act, which took blood levels of lead for all Americans down to a fraction of what they had been. “Putting the two together,” she says, “it seemed that this big change in people’s exposure to lead might have led to some big changes in behavior.”"
The economist, Jessica Wolpaw Reyes, of course, did attempt to hold all other factors constant. But only more research will show if she is right.