Believe it or not, the ratio of boys to girls born is not 1:1. It varies slightly throughout the world, but is generally around 103 or 104 to 100.
Demography is the study of human numbers and is about 300 years old. During its early development, the popular explanation for the world's gender imbalance was based on religious ideas: e.g., God foresaw boys being more likely to engage in mischief and potentially dangerous behavior, thus more likely to die before childbearing age, and therefore needing a little extra advantage from day one.
Modern theories tend to avoid speculation about divine intervention. Yet they still must acknowledge the fact that indeed more boys are born than girls, and indeed that imbalance seems to correct itself by the time both sexes finish puberty.
The world's variations in birth sex ratio is associated with many factors. Ethnicity, parity (or birth order), and age and health of the parents seem to affect the number. A topic of debate is whether the sex ratio of the current adult population might also affect that of the next generation.
Biologists have confirmed that sperm carrying the Y chromosome (which produces a male) is slightly more robust than those carrying the X chromosome (for females), and therefore stands a better chance of fertilizing an egg. The evolutionary reason for this, however, is still largely unknown.