I once confided to my brother that I have the habit of watching lines in the ground as I walk and not stepping on them.

Now, this is certainly compulsive, but not obsessive. If I have something on my mind or am walking with someone, it never occurs to me to look, but left to my own thoughts, I'll match my stride to the pattern of lines on a sidewalk as I walk along it.

To my surprise, he said he did the same, but had never really mentioned it to anyone. At the time, we decided that perhaps it was just one of those odd family quirks, but when we asked our respective groups of friends about it, lo and behold, there was a pattern.

About 1 in 3 of our friends did this or something like it. Some said that they would look at the regular spacing of trees or parking meters. Others said that they tried to step ON the lines instead of avoiding them. Everyone who did admit to crack-watching was in some sort of technical field like a programmer or scientist.

The opposite correlation also seemed to hold true. Our less-technical friends, people who disliked computers/science, tended to not have any clue what we were talking about. These friends tended to be in people-related professions, like customer service or marketing.

Now, it's pretty obvious where crack-watching comes from. It's the brain's tendency to find patterns in otherwise unimportant sensory input (think Jesus's face on a zuchinni) but not everyone feels compelled to hunt for and react to more abstract patterns like lines on a sidewalk.

The correlation with math/science/computers must be that people with these sort of brains like the abstract patterns you find in technical subjects.

This line of thought had settled the issue so far as I was concerned until I read an article about the increased incidence of Autism in silicon valley. Specifically, the article had a sidebar on Autism that lists paranoia about stepping on cracks as being a minor symptom. It went on to describe the milder form of Autism called Asperger's Syndrome that is mild enough to go undiagnosed in many children. It was a spectrum disorder.

Hmm. Spectrum...

Autism
Asperger's
Crack-Watchers
Average People
Very Sociable People
???

Watching Cracks in the Sidewalk