Is the firstborn son biologically different in any way from the sons that follow?

Perhaps his epigenome is different? I have heard that a man's testosterone levels decrease when he becomes a father -- could this then have implications for the epigenetics of subsequent sons?

  • $\begingroup$ King Joeffrey was a monster. His younger brother, King Tommen, was a sweetie. Jamie is a heartthrob. Tywin somewhat less so (as a dwarf.) So yes, of course sons are different, but not in any predictable way. (Sorry to use fiction to answer, but it's as accurate as anything else on this matter.) $\endgroup$ Commented Aug 11, 2017 at 1:56
  • $\begingroup$ This report from 1 million babies finds a negative effect on health at birth for lower parity offspring: cinch.uni-due.de/fileadmin/content/research/workingpaper/…. Whether that is due to biological characteristics of the babies themselves or the mother is not clear. $\endgroup$
    – vkehayas
    Commented Oct 10, 2017 at 12:37
  • $\begingroup$ The firstborn son has equal chances of birth asphyixiation, being born blue, and having negative traits like autism, as the other children, based on the egg-sperm combination. The quality of the gametes of a traditional female mother from age 16 to 30 let's say, does not vary that much. $\endgroup$ Commented Dec 9, 2017 at 12:29

2 Answers 2


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.

  • $\begingroup$ There are apparently downvotes on this. I realise there are non-scientific links but I only provided them for completion. If anyone has a problem with the answer they should provide a constructuve comment with their vote. $\endgroup$
    – Joe Healey
    Commented Aug 11, 2017 at 23:27
  • $\begingroup$ It would be more representative of the literature to also report the magnitude of the detected IQ effect, which was roughly 1.5 units in this larger study: pnas.org/content/112/46/14224. $\endgroup$
    – vkehayas
    Commented Oct 10, 2017 at 12:23
  • $\begingroup$ I expect that 1.5 units probably derives from the cultural effect of competition and favouritism, and the higher likelihood of the the other children to have depression. $\endgroup$ Commented Dec 9, 2017 at 12:31

I remember reading/hearing something about the effect of older brothers on the chances of people becoming certain types of people.

It went something like the oldest was most likely to be a pilot/doctor/CEO etc.

The second was most likely to be a teacher/middle management etc.

There's plenty of blogs that suggest the more older brothers a man has, the more likely they are to be homosexual. But this obviously doesn't look at whether or not Biological influences lead to this "birth order effect", or cultural influences.

I remember a suggestion that it was to do with antibodies (or some other substance) that builds up in the mother's womb after having son's. But again I can't find any evidence for that, so it might be pure speculation.

NB. I don't subscribe to this theory, but if you're really interested, it might be worth looking at the science behind it.


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