Is the sex of a baby a fair coin toss, independent from one birth to the next, or can some parents be more...

probability/sex.ratio New

New entry, edited from material collected for the archive and not yet in a released version.

Problem

Is the sex of a baby a fair coin toss, independent from one birth to
the next, or can some parents be more likely than others to have boys
(or girls)?
[ Editor: problem statement reconstructed; the original question, from
a thread "Scientific questions that are not necessarily puzzles", is
not in the collected postings. ]

Show solution

Instead of just guessing like others, I actually spent five minutes
checking whether there was any data on this.  Synopsis: Various factors
seem to be able to influence the sex of the child at birth, and some of
these will be consistent between births for a particular pair of
parents.

Specifics:

Epidemiology 4:549--HLA compatible parents may have a high male-to-
   female ratio (and more SIDS)

Social Biology 39:285--There is a positive correlation between human
   sex ratios and caloric availability, due to a higher mortality of
   male embryos due to nutritional defects

Human Biology 64:523--Still borns are mostly males

J Theo Bio 155:121--Some evidence supports sex ratio control by the
   hormonal level of both parents at the time of conception, which can
   be influenced by such factors as time of insemination during cycle,
   parental occupations, parental temperament (eg female hierarchy
   position in polygamous societies), etc.

-- Daniel Mytelka <mytelka@mendel.berkeley.edu>

[ Editor: see also probability/family, which assumes boys and girls
are equally likely. ]