Is there any biological basis for the emphasis on the value of the firstborn son?
There are lots of cases reported now that suggest that overall the firstborn child is usually more intelligent. The articles supporting this are based on behavioural and economic study though, so the biological basis is a little lacking.
It's also not clear if this is nature or nurture. The firstborn may benefit from their parents undivided attention disproportionately.
Here's the press release for it. And the coverage from the Independent
and Forbes
I read somewhere that the firstborn is more likely to be overweight too, but I can't find the article now. Other people may know other traits that are supposed to differ between children.