When you suffer stress in pregnancy, the risk of abortion affects male fetuses more than female fetuses, which explains that in periods of crisis, fewer men are always born. This is the work of Ralph Catalano and Tim Bruckner of the University of California and is published in the electronic edition of Proceedings of the National Academy of Sciences.
Apparently, male fetuses survive less than female, because according to the study of these researchers, factors that are caused by stress such as economic crises or natural disasters, reduce the birth of male children. For this selective abortion, there are two theories, one of which is that natural selection has a mechanism for future moms, in situations of hardship, to improve their grandmothers' future by selecting the sex of the child, since by not concluding the pregnancy, the mother is available to breed a new fetus.
The other explanation is that it is unlikely that the maternal organism influences the selection of the sex of the newborn. Catalano and Bruckner propose as a more plausible explanation that the stress that affects the future mother mainly affects the weakest and of them, the male rather than the female.
For this study, data from the Swedish population were used, as they have one of the largest demographic series in the world.
