The human sex ratio is skewed toward males, and this is the subject of much research. A review of some of the theories currently available was conducted, searching and analyzing the current scientific literature. Suggestions to explain the ratio include unequal proportions of X and Y sperm, the effect of maternal diet, maternal testosterone levels, and the natural fluctuations in the consistency of the cervical mucus. Although the difference in X/Y sperm proportions does not adequately explain all the data, the other theories do seem to explain the greater percentage of male births. However, there is still not enough information available to verify which theory is the most plausible.
Cofsky, Y. (2012). Current Theories on the Human Sex Ratio. The Science Journal of the Lander College of Arts and Sciences, 6(1). Retrieved from https://touroscholar.touro.edu/sjlcas/vol6/iss1/11