The fallacy of questionable classification is not one of the ones discussed most commonly in critical thinking classes and the like, but it actually occurs quite often and is worth thinking about.
To commit the fallacy of questionable classification means to treat, or make claims about, one group in a way that reflects a confusion between it and some overlapping group.
Let’s clarify with an example. It has long been common for people to say “AIDS is a gay disease,” or “Gay people are at high risk for AIDS,” etc.
But is it really being gay that’s the relevant factor here? AIDS is transmitted through an exchange of bodily fluids, often through sexual activity (though other things, such as blood transfusions, also can transmit the virus). Acts that tear tissue so as to cause bleeding can be especially risky in this sense, so anal sex without a condom would be up near the top of the list. Of course whatever the risk is, you compound it the more you engage in activities that exchange bodily fluids. So a person is at greater risk if they have a lot of unsafe sex with a partner who has or might have the virus in their system, or if they have a little unsafe sex with a lot of partners who have or might have the virus in their system. (And if they have a lot of unsafe sex with a lot of partners who have or might have the virus in their system, then their risk is off the charts.)
In a lot of people’s minds, they then associate this with being gay. They think of bath houses and what they’ve heard about gay promiscuity and such, and so they start treating being gay interchangeably with engaging in behavior that is high risk for transmitting AIDS.
But in order for this not to be an instance of the questionable classification fallacy, there would have to be a total (or at least close enough to total for practical purposes) overlap between the categories of “gay person” and “person who engages in behavior that is high risk for transmitting AIDS.”
And there isn’t. Not even close. Plenty of people fit in the first category and not the second; plenty of people fit in the second category and not the first.
It doesn’t matter, by the way, if there’s more overlap between, say, gay people and people who engage in this behavior versus non-gay people and people who engage in this behavior. If you’re going to speak of two categories as if they’re the same, they need to be the same. The fact that something else is even more dissimilar is irrelevant.
Because if you equate these two categories, you’re in effect mis-categorizing celibate gay people, monogamous gay people who practice safe sex, etc.
Heck, lesbian sex ranks well below even most heterosexual sex in terms of riskiness, yet somehow lesbians are getting clumped into this same category.
Stereotypes are indeed a prime source-though not the only-of the questionable classification fallacy. It’s common for people to make statements about black people that imply most or all of them are welfare recipients, or statements about white Southerners that imply most or all of them are ignorant racist hillbillies, or statements about prisoners that imply most or all of them are hardened criminals guilty of murder or similar horrific crimes. But in each case, some are and some aren’t, so you commit a fallacy when you equate them as if all are.
To guard against the questionable classification fallacy, it’s important to be careful when making sweeping general statements. Be sure when you say something about a group that you aren’t really talking about a subset of that group, or some overlapping group.