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On average, about 105 males are born for every 100 females. The explanation for this imbalance lies in the greater mobility of spermatozoa that carry the Y chromosome, which defines the male sex. The curious thing is that recent scientific studies have revealed that, in an average ejaculation, there are more spermatozoa with the X chromosome, which determines the birth of girls, than with the Y. But, as “female” spermatozoa are slower, their chances of fertilize the egg are smaller, paving the way for the birth of more males. This initial mismatch, however, is mitigated throughout life. “There is a higher mortality of men than women from birth, especially between 18 and 35 years old”, explains demographer Cíntia Simões Agostinho, from the Federal University of Minas Gerais (UFMG). The consequence of this is that the global population average is 101 men to 100 women. And in many countries, including Brazil, there are more women than men.