95 Danish and Japanese scientists have found that whether young parents smoke is related to the ratio of males to females giving birth to babies. If neither parent smokes, the ratio of males to females is 1.214 to 1. If both parties smoke more than 20 cigarettes a day The ratio of male to female babies decreased to 0.823 to 1. Even if only one party smoked and the smoking habit was not heavy, the probability of giving birth to a boy would decrease. It is speculated that the sperm or male embryo carrying the chromosome is more vulnerable to smoking. In addition to smoking, emotional stress and environmental pollution can also cause a decline in the proportion of boys. From this perspective:. ['X chromosomes are not affected by smoking, so there is a greater chance of having a girl.', 'There are more men than women in the world because non-smokers are the majority.', 'Both spouses are less likely to have a baby than both boys.', "Men's Z chromosome is more fragile than X."]