Don't know the ins and outs but yes its physiological.
And something to do with emotional development also.
I think it also demonstrates the difference between the sexes. People say treat all equally etc but there are difference, girls and boys will and do have differences no matter how much we give barbies to boys and action men to girls there are still differences.
I think subconsciously girls become more "aware" and a wanting to feel clean and dry, which comes later in many boys.
Before anyone jumps down my throat i am all for equality with toys and bringing up of girls and boys. i just feel nature is telling us there is a difference and we can't argue with that.
Ds took quite a while to get it - I have put this down to him not wanting to be bothered with stopping playing to go and do a wee - why should he when he could just do it as he played. The turning point was when we went on holiday with lots of our (and his) friends, and he didn't want to be seen to be a 'baby' who had accidents.
They don't, always. My dd wasn't dry until she was almost 3, and at almost 4 has only just stopped having a nappy for a poo. She's still not dry at night. Her best friend, also a girl, was also still in nappies until she was almost 3.
My friend's two sons were both dry day and night by the time they were 18 months old.
My ds's were much easier to train than my dd's! The boys were done & dusted at just over 2yo while the girls were 2.5+yrs and took forever to get the idea. Mind you, the boys were in cloth nappies while the girls were in disps. I wonder if that is what made the difference?
Would agree with sue my DS was much easier than DD. DS didn't wet himself after he was trained where as DD still does now and shes 3. DS was also dry at nights at the same time by himself where as we have had real trouble with DD.