Back in the good old medieval days, queens had one job: producing heirs, which meant sons. Sons were so important that King Henry invented charges of treason against his second wife, the infamous Anne Boleyn, because she couldn't produce a son. With the discovery of DNA and the Y chromosome, it became clear that Henry had no one to blame but himself: because males have both an X and a Y chromosome, the sperm determines the sex of the child. Females are XX, and thus can only donate an X chromosome to the egg. A very tidy explanation with much vindication for poor Anne Boleyn, but is sperm the whole story?
A recent study from the UK showed that women who had higher caloric intake around the time of conception are more likely to have boys than girls. This sex bias based on environmental conditions has been reported in many other animals, including mammals like horses and cows, but this the first report of it in humans. Granted, it is not a huge difference: 56% of the high calorie group had boys, versus 45% of the low calorie group. The article also does not say if this is a statistically significant result.
I find this interesting because they don't know how it works. Obviously, the male determines the actual sex of the baby, but the mother's womb can favor the development of one sex of fetus over another. Researchers knew from in vitro fertilization treatments that high glucose (simple sugar) levels favors male fetus development and discourages female development, so that might be a link. Fortunately, if they really want to look into the mechanism, human studies will probably be unnecessary. Many mammals change the male: female ratio based on the environment.
Basically, males are a risky evolutionary bet. They can produce more offspring if females are in ample supply, but if there are more males than females, only the best males will reproduce and the rest are left without offspring. Females have a more limited reproductive capacity, but can basically always reproduce (it's just not that hard to find a willing male). So, if the environmental conditions are good, say by having ample food supply, a male child is more likely to grow up strong, fit, and ready to spread some genes around. If there is a low food supply, males are less likely to be successful, and females are a safer bet.
Personally, I'm quite excited to see what the results of a more detailed study are. The glucose explanation makes sense, but how does the uterus determine what chromosomes a fertilized egg contains? Also, 56% is not a large difference, so is this result "real" and what other studies will be done to confirm that?