I did read some summarized research once that talked about gender selection being paternally based--I mean, obviously, but the father's father. That some men are inclined to make more of one type of sperm than another. In my dh's family there are 10 children, 4 males, 6 females. If his parents had stopped at four children, they'd have had 3 males, 1 female--with 3 boys to start. As they enlarged their "N"
it turned out that they were 60% female. Amongst him and his four brothers there are 14 children. Of the 14, 9 are female, 5 male. Of those five males, three are firstborns. The tendency in dh's family is to have a male born first, then a female. You can see that it closely matches the family of origin's stats though. To be fair, my family is primarily female as well. Of the 7 grandchildren, four are female.