In general, more male children are born than males. If I remember correctly conceiving a male is about 1% more likely than a female. This is because the y chromosome is smaller than the x chromosome, which makes male sperm slightly lighter and faster. I have also read that it is the case than when a womb contains both a male and female twin the male tends to take up more nutrients and be born stronger and healthier than the female. (Two male twins compete more though, and end up less healthy than 2 female twins.)
Apparently one verifiable prediction of Evolutionary Psychology is that more physically attractive people are more likely to have daughters. This is thought to be because physical beauty is more important in deciding whether a female can reproduce while wealth matters more for males. Most people are not all that attractive though, so for them producing more males is preferred.
In antiquity it was not uncommon for unwanted children to be abandoned. In Roman society the child's life was not considered to have value until the father acknowledged it as his own. A newborn would be laid in front of the paterfamilias and not given care until he picked it up. He could instead opt to leave the child exposed somewhere where in theory others could come to adopt the child, but in reality it would probably die. This practice was not uniquely Roman, but very similar to customs found throughout the Mediterranean. The Romans considered the Jews and Germans to be barbaric for not practicing such infanticide, as it was considered cruel to allow deformed children to weaken the gene pool and drain resources from those who were born healthy and would become productive members of society. As the societies of antiquity were largely, sons were valued much more than daughters and so daughters faced a greater risk of being abandoned and dieing young.
The Spartans were unique among the Greeks for abandoning far more males than females, as their physical standards for suture warriors were very high. We tend to think of them as only warriors devoted to Ares, but they were actually just as devoted to Eros and were very concerned with physical beauty. Their women were especially famed for their appearance, both natural beauty and physiques maintained through the sort of regular exercise considered unfeminine elsewhere. It is no accident that Helen of Troy was believed to have been a Spartan. Their physical beauty and custom of being harsher on males made theirs a largely female dominated society. While most Greeks kept their women indoors and considered it unseemly for a respectable female to visit the marketplace, commerce was conducted almost solely by women in Sparta. Their downfall was because they could not maintain a large enough male population to protect themselves.
Also, I read a long time ago that an increase in the proportion of male children born tends to be associated with a period of warfare once those children become adults.
