[thedoc (OP)]

Claes Gauffin asked the Naked Scientists:
   
How come most of us humans have a predilection for crossing our legs when we sit? The habit seems pretty universal, but it doesn't seem to serve any useful purpose. In fact people with hip joint replacements are advised not to. Yet it is uncannily hard to avoid doing it once you get the habit.

What do you think?

[majorminor]

It is comfortable would be somewhat obvious and inadequate answer.  Maybe in doing so we are changing position ... stretching  muscles, increasing blood flow. Perhaps psychological  almost like we are giving ourselves leg hugs. I just do it because it is a learned behaviour from kermit the frog  when I was watching sesame street as a child.

[cheryl j]

Well I just did it and tried to pay attention to its effects. It does seem to stretch certain muscles in the hip. The weird thing is - why is it more comfortable to cross your legs or arms or fingers one way but not the other? In fact if you try to do it, you almost have to think about how to do it. It feels quite weird.

[menageriemanor]

Women of a certain age, especially wearing skirts/dresses were taught to do it, automatically, and when young I did, you had to cross, then slant them elegantly, but being short I never pulled off elegant in my life.  Now I'm short and stout and old, and never knowing when I'm going to be climbing through fences, I'm often in trousers, and never think about it. I can't nonchalantly throw a leg over the other.  I'd have to force one over the other, and tap it down with a hammer...   I do rather feel a glimmer of jealousy, at a long, lean thigh.  Just get me onto 1/2 empty top shelves in supermarkets, where I have to beg the use of a passing husband to get the no grain cat biscuits.

I noticed the Queen doesn't cross her legs, and now can't recall if she did, when young.  She has my same truly depressing shortness of leg.

[dlorde]

Postures are often used as social signals. Crossed legs suggest reserve or restraint, concealing the crotch (unless the intent is to display the upper thigh, e.g. in short skirt, as a provocative sexual signal). Young males competing for mates often sit legs akimbo, ignoring social niceties, signalling strength & fearlessness, and possibly sexual intent.

[sagopalm]

Very glad to encounter this thread. I have been asking the same question and trying to get a satisfactory answer for a few years.

My answer is that crossing legs makes a more stable position to hold. I used word stable instead of comfortable here since stable is a basic physics concept and comfortable is the result of a stable position felt by the human. This is the first time I post it online:-)

[jerrygg38]

Good question, never thought about it. We often eat in the living room on small individual tables. My daughter gets upset when I cross my legs and she sees my right shoe. So she asks me to cross my left leg. But this hurts more. I cannot do a full cross as it is a little painful on the right knee joint.
  So I tried an experiment to find out why humans cross their legs. The answer is life and death. Sit down and try to stand up rapidly to escape a predator. It takes a certain amount of time. Now do the same experiment and cross your leg. Notice that the downward motion of the leg enables you to stand up more quickly and escape from the animal or human trying to kill and eat you. So the answer is life and death. Those who cross their legs will survive and those who do not cross their legs will be eaten more often.