My 5 year old would like to know the answer to this.  Is it the same for other birds too?  We can tell some sexes of ducks by their plumage, but do the birds do so in the same way?

As the the proud father of two young boys I feel inclined to quit whilst I'm ahead. My wife semi-jokingly says that she would like a third child if she could be guaranteed a girl. My argument for not pursuing this "opportunity" is that there are quite a lot of all-male siblings in both sides of our extended family and therefore the chances are we would have another boy.

With the recent news from the Beckham household too, I wondered if there was any genetic basis for this?

Finally, what would be the evolutionary advantage of single-gender family units? The benefit of accumulated parenting experience with one gender?

At birth, take a male and a female human from each of their parents.

Raise them together, but independent of any possibility of social learning - other than what they get from interacting with each other.

I.E they aren't taught a language, there are no other humans around, and thus no pre-existing social conventions to mimic.

And here's the question: Do they know what sex is and how to do it?

It's almost inevitable that they will independently discover:

1. touching their reproductive organs feels good,  
2. they are "attracted" to the opposite sex 

But there's a rather large leap from that, to:

1. the male innately knowing that he ought to penetrate the female until climax, and 
2. the female understanding that when that happens, she will get pregnant and produce offspring. 

Are there any studies that have tested this on mice or some other species?