Scientists have discovered that women on low-calorie diets or who skip breakfast at the time of conception are more likely to give birth to girls than boys. It also shows that child's sex is associated with the mother's diet, and higher energy intake is linked to males.

There has been a small but consistent decline, of about one per 1,000 births annually, in the proportion of boys being born in industrialized countries over the past 40 years.In humans, going without breakfast may be interpreted by the body as signaling low food availability, since it depresses levels of blood sugar.

Although sex is genetically determined by fathers, it is known that high levels of glucose encourage the growth and development of male embryos while inhibiting female ones, although the exact mechanism is unclear. In evolutionary terms, this correlation may make sense.

Males' breeding potential is strongly influenced by fitness, while females breed 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," Mathews said.

Some researchers have been sounding alarms for years over the change in sex ratios in developed countries and have in the past blamed pollutants and synthetic chemicals such as those found in some pesticides which disrupt human hormones.