Although sperms carrying Y-chromosomes greatly outnumber those carrying only X-chromosomes, and thus the number of male foetuses conceived is considerably more than the number of female foetuses, a far higher proportion of male foetuses abort (one survey showed between 137 and 142 males aborting at the 20th week of pregnancy compared with 100 females). Nevertheless, there are still slightly more male births than female (between 104 and 107 males for every 100 females). Despite this, the sex ratio in many countries shows a higher percentage of females to males, due both to a higher mortality rate for male than for female infants and to deaths incurred by wars. This may lead to a dearth of males to carry out traditionally male tasks and to a large number of unwillingly celibate females; and also create a burden on those societies where the male population determines its strength. It may also lead to a higher incidence of divorce. Conversely, where the sex ratio shows a higher percentage of men, it is felt that the society might be more aggressive, with a higher incidence of crime and of racial and social tensions.
An additional factor is the present possibility of determining the sex of the baby before birth and the future possibility of choosing the sex of the baby before conception. These possibilities may lead to some advantages - the eradication of sex-linked genetic disorders, for example, such as haemophilia, some enzyme deficiency diseases and a form of muscular dystrophy, all of which are many times more common among males than among females. But because of the increasing availability of legal abortion and the widespread preference in many cultures for the birth of sons rather than daughters, some demographers fear that the sex ratio may be further imbalanced by abortion of female foetuses and the choice to conceive sons.