Differences of Degree vs. Differences of Kind

Just wanted to jot down a principle I've come across a couple places and tends to explain a lot of the erroneous (although often well-intentioned) paths we go down: people often mistake (or elevate) differences of degree for difference of kind. From the NYTimes Magazine, an article on gender-based education covers the merits and demerits of single-gender classrooms. In it, an example:

One reason for this, Giedd says, is that when it comes to education, gender is a pretty crude tool for sorting minds. Giedd puts the research on brain differences in perspective by using the analogy of height. “On both the brain imaging and the psychological testing, the biggest differences we see between boys and girls are about one standard deviation. Height differences between boys and girls are two standard deviations.” Giedd suggests a thought experiment: Imagine trying to assign a population of students to the boys’ and girls’ locker rooms based solely on height. As boys tend to be taller than girls, one would assign the tallest 50 percent of the students to the boys’ locker room and the shortest 50 percent of the students to the girls’ locker room. What would happen? While you’d end up with a better-than-random sort, the results would be abysmal, with unacceptably large percentages of students in the wrong place. Giedd suggests the same is true when educators use gender alone to assign educational experiences for kids. Yes, you’ll get more students who favor cooperative learning in the girls’ room, and more students who enjoy competitive learning in the boys’, but you won’t do very well. Says Giedd, “There are just too many exceptions to the rule.”

Also, in Clay Shirky's recent (and excellent) book, he discussess this as well. I come back to the idea of what fantastic differentiators we are as human beings, sometimes to our detriment. We focus far more on what makes us different (skin color, religion, culture, language, etc.) when we're vastly more alike than not.