Following reading your post and replying, I spent a couple of minutes googling, seems theres been some interesting research since my original post.
Will have to do some more reading, thanks for waking up my grey cells.
A little titbit to leave here: https://www.npr.org/templates/story/sto ... Id=4758495
KNOX: Many things could influence that environment. Dr. Karen Norberg of Washington University came up with the single-mother hypothesis. It's based on 80,000 births over 40 years. She found that boys were less likely to be born to mothers who were not living with a man at the time of conception.
KNOX: If a woman had a child without a live-in male partner, the odds were tipped slightly against having a boy. For children born later to the same mothers when fathers were living in the household, the odds of a boy were higher. Some listeners wondered if abortion could be a factor. Norberg says she found the same pattern prior to Roe vs. Wade. Some think environmental chemicals like pesticides are likely to be a more important factor. Pete Myers of Environmental Health Sciences is a leading exponent of that theory. He says cases in Italy, Russia and Canada suggest certain chemicals that disrupt hormones reduce the proportion of boy babies.
Dr. NORBERG: There's been a very dramatic rise in the number of women who are single at the time of a child's birth and not living with a male partner.
KNOX: From 5 percent in the 1940s to more than 20 percent today. But one thing the numbers can't tell us is how would a woman's living situation affect a baby's gender. Norberg thinks it has to do with feast and famine and family resources acting over thousands of generations. It takes about 10 percent more calories to gestate a male fetus, and it probably takes more food to raise a boy to adulthood.
Dr. NORBERG: Yes, boys cost more than girls, and if the single mother sort of almost by definition only has her own resources to put into rearing the child, it may be more risky for a single mother to be undertaking the rearing of a son than a daughter.
KNOX: Norberg doesn't think it's necessarily good or bad the sex ratio is changing. It just shows that humans adapt to their circumstances in surprising ways. Richard Knox, NPR News, Boston.
One article on npr... just struck me as interesting, first few results on 'environmental influences gender pregnancy'
Theory and data suggest that a male in good condition at the end of the period of parental investment is expected to outreproduce a sister in similar condition, while she is expected to outreproduce him if both are in poor condition.
Accordingly, natural selection should favor parental ability to adjust the sex ratio of offspring produced according to parental ability to invest. Data from mammals support the model: As maternal condition declines, the adult female tends to produce a lower ratio of males to females.
which leads to:
https://en.wikipedia.org/wiki/Trivers%E ... hypothesis
some interesting reading coming up, thanks again
A Sex Allocation Theory for Vertebrates: Combining Local Resource Competition and Condition‐Dependent Allocation