Mother's Diet Influences Infant Sex: High Energy Intake Linked To Conception Of Sons:

...The study shows a clear link between higher energy intake around the time of conception and the birth of sons. The findings may help explain the falling birth-rate of boys in industrialised countries, including the UK and US. The study focused on 740 first-time pregnant mothers in the UK, who did not know the sex of their fetus. They were asked to provide records of their eating habits before and during the early stages of pregnancy. They were then split into three groups according to the number of calories consumed per day around the time they conceived. 56% of the women in the group with the highest energy intake at conception had sons, compared with 45% in the lowest group. As well as consuming more calories, women who had sons were more likely to have eaten a higher quantity and wider range of nutrients, including potassium, calcium and vitamins C, E and B12. There was also a strong correlation between women eating breakfast cereals and producing sons.

What's going on? Probably the Trivers-Willard Hypothesis:

Dr Fiona Mathews said: "Potentially, males of most species can father more offspring than females, but this can be strongly influenced by the size or social status of the male, with poor quality males failing to breed at all. Females, on the other hand, reproduce more consistently. If a mother has plentiful resources then it can make sense to invest in producing a son because he is likely to produce more grandchildren than would a daughter. However, in leaner times having a daughter is a safer bet."

The phenomenon of high status females producing sons and low status females biasing their offspring toward daughters is well known. Usually one doesn't need to make recourse to a prenatal process; infanticide or neglect after birth can explain the imbalances in most societies. With the spread of elite values in many "traditional" cultures the son-preference has been generalized, at least in the ideal, but one needs to be cautious about projecting back to the past.^1 There is the famous finding that medieval cemeteries show more male infants who died early than female among the peasants, and the inverse among the nobility. The paper is being published by The Proceedings of the Royal Society: B, but is not on their website. 1 - In India most low caste groups practiced brideprice until the 20th century, when they switched to dowry in imitation of the high castes.