A parent's report about a child's behavior is certainly subjective, ” Engel said.

Some effects were stronger in boys than girls, but the associations to the chemicals were still considered significant in the girls, Engel said.

Or if a household does underreport its spending, he assumes that it also downplays its food expenses proportionately, so that their Engel’s coefficient is unaffected.

So it’s possible that two families with the same Engel’s coefficient really do have different incomes, because of where they live, their family circumstances or their gastronomic passions.

But there’s still plenty of play in Engel’s law that he cannot explain or control for.