Autism is another case in point. There has been a significant increase in reported cases of autism in recent years. However, as numerous organizations, including the Centers for Disease Control, the American Medical Association, the Institute of Medicine, and a special court associated with the U.S. Court of Federal Claims have found repeatedly, there is no credible evidence that thimerosal-containing vaccines cause autism.
This is obviously a very complex topic, and the reasons for the reported increase remain elusive.
Nevertheless, many experts attribute at least part of the increase to two factors: 1) a widening of the definition of autism to include Asperger's syndrome and a variety of other "autism-spectrum disorders" and 2) a keener awareness of these disorders and a more vigilant search for them.
The incidence of human trafficking is also dependent on (changes in) the definition. The strict definition codified in the 2000 U.N. Palermo protocol requires that an unwilling person, whether by fraud or coercion, be transported for the purposes of exploitation, either sexual or labor-related (both very serious crimes). This statement seems clear and literal.
But there are other definitions, one of them the U.K.'s 2003 Sexual Offenses Act, in which trafficking includes the voluntary (i.e., no coercion, trickery, or force) travel of adult sex workers looking for a higher income. The law has some much needed protections, especially for children, but if interpreted sufficiently loosely, it may encompass all forms of prostitution.
Using the strict definition of trafficking, the relatively few successful prosecutions suggest that the number of such cases in the U.S. and U.K. is much, much lower (probably by an order of magnitude or two) than many of the estimates that have been published. The U.K. Act, especially its very loose interpretation, probably derives more from a moralistic aversion to voluntary prostitution than to an effective, focused effort to put an end to trafficking and sexual slavery.
Of course, redefinition isn't always wrong, as long as its proponents are clear about what they're doing. Thus, "Define your terms" is one of the most basic pieces of advice given to those interested in constructing logical arguments. "Don't equivocate" is a related suggestion for good logical hygiene.
That is, don't start out talking about the incidence of stomach cancer and then later say that stomach problems are becoming epidemic without making clear that you're now including food poisoning as a stomach problem.
Unfortunately, too many people, among them politicians and zealous activists of various stripes, ignore the above commonsense advice and instead follow the counsel of Humpty Dumpty. And we all know what happened to him.
John Allen Paulos, a professor of mathematics at Temple University in Philadelphia, is the author of the best-sellers "Innumeracy" and "A Mathematician Reads the Newspaper," as well as, most recently, "Irreligion: A Mathematician Explains Why the Arguments for God Just Don't Add Up." He's on Twitter (twitter.com/johnallenpaulos), and his "Who's Counting?" column on ABCNews.com appears the first weekend of every month.