“Abnormal” is not an insult. And “normal” is not a compliment.

People bend over backwards these days to avoid calling anything “abnormal.” Apparently they think it might hurt someone’s feelings.

For example, Robert Plomin’s recent book about human DNA argues that because thousands of small genetic differences affect human psychological traits, “there are no disorders, that the ‘abnormal is normal’.”

In other words, he argues that since we’re all a little crazy, there’s no such thing as being crazy. The abnormal is normal.

But what’s normal simply means what’s common. Abnormal means uncommon. It isn’t normal, unless we’re going to throw out the dictionary and use words meaninglessly.

Words should mean what they mean. I care about things like that. It’s one of the ways I’m abnormal.

Most human traits vary through the population in what’s called a “normal distribution.” You can graph it in a bell-shaped curve, as shown in the picture at the top of this blog post.

At the highest point of the graph, in the center, is the average (also called “the mean”).

For example, suppose you’ve got five people whose heights are 60 inches, 65 inches, 70 inches, 75 inches, and 80 inches. If you add up all the numbers, you get 350 inches. Divide that by the number of people (5), and you get the average height: 70 inches.

But what if you’ve got another group of people whose heights are 35 inches, 40 inches, 70 inches, 100 inches, and 105 inches? The average is still 70 inches, but it’s much less helpful because of the large difference in heights.

The same problem arises if the heights are 70, 70, 70, 70, and 70. The average is still 70, but there’s no variation at all in the people’s heights.

If you only looked at the average heights, you’d think all the groups were pretty similar.

Standard deviation is a way to measure how much a trait varies.

For the first group (60, 65, 70, 75, 80), the standard deviation shows some variation: about seven inches. For the second group, it shows a lot of variation: 29 inches. For the third group, it shows no variation: zero inches.

Standard deviation isn’t very helpful with small groups. But if a population of a million people had an average height of 70 inches with a standard deviation of seven, then:

• 68 percent of people would be within one standard deviation of the average.

About 680,000 people would be between 63 and 77 inches tall.

•   95 percent of people would be within two standard deviations of the average.

About 950,000 people (which includes the 680,000) would be between 56 and 84 inches tall.

• 99.7 percent of people would be within three standard deviations of the average.

About 997,000 people would be between 49 and 91 inches tall.

That leaves at most 3,000 people out of a million who are under 49 inches tall or over 91 inches tall. They are abnormal. It doesn’t mean they’re bad, stupid, or that their lives are worth any less than the lives of normal people. It means only that their height is far out of the normal range.

For evaluating them as people, their height is completely irrelevant. On the other hand, if you’re deciding what sizes of clothing to manufacture, the normal range of height is very relevant. You want to make sizes that will sell enough to make a profit. Those 3,000 people will have to find special stores to buy their clothing, or order it online.

For individual success and social harmony, what’s normal matters in two ways:

• Our normal ideas and institutions have developed over thousands of years in many different kinds of human societies. They’re still around because they worked for our ancestors and their societies. Abnormal concepts and  ways of life might work or they might not work. We don’t always know, and the penalties for being wrong can include demographic extinction. Caution is appropriate.
• Like lower animals, we tend to divide into groups based on cues such as appearance and behavior. If we see some people as abnormal, we are more inclined to see them as non-members of our own group. That can lead to social conflict. That seems to be Plomin’s motivation for denying that abnormality exists: “The most general implication of this view of the abnormal as normal is that there is no ‘us’ versus ‘them’.”

Plomin’s approach is well-intentioned, but I think it’s ultimately self-defeating to deny that human differences exist. If we pretend that they don’t exist, they’ll still be there anyway. We will still be “us” and they will still be “them.” But we won’t be able to do anything to mitigate the differences because we’re pretending they don’t exist.

Our challenge is to work with our differences in peaceful and socially beneficial ways. It’s not easy, but we can do it.

Check out my new book Why Sane People Believe Crazy Things: How Belief Can Help or Hurt Social Peace. One reviewer said “Using a dash of humor and an accessible style of writing, this will delight fans of books like Yuval Harari’s Sapiens. Highly recommended.”