I've always figured my family was just a statistical anomaly. My sister has looked into various chemistry or dietary reasons for this (wanting to have a boy next), but the research I've seen on this has generally been pretty sketchy and incomplete, so I've decided to chalk it up to chance -- at least, until I see compelling evidence otherwise.
Well, I read an interesting article today that seems to indicate that there are some chemicals out there that do noticeably tip the scales. I've long known that the benzene and other chemicals in sunscreens mimic estrogen, shifting the sex of fish toward the "default" female, even during an individual fish's life, and having similar effects in rats and other animals. I have blogged about the cosmetics safety database listing the safety of sunscreens. (It looks like the famous "wear sunscreen" column, in which the author said the only sure advice she could give was to wear sunscreen, might have been wrong after all.) I had also heard that PCBs, common in flame retardants and coolants, and other chemicals had documented effects on development, especially sexual development. But this is the first I've heard of the chemicals actually causing a shift in sex ratios at birth.
According to the article, the Arctic populations that the Arctic Monitoring and Assessment Program studied had imbalances that were particularly pronounced (twice as many girls as boys, where before the ratio was 1.1 boys for every girl) because of their reliance on marine meat, which is high on the food chain and thus had heavy concentrations of these chemicals.
Though it's doubtful that anything like this has caused the generations-long surplus of girls in my family, it does make me reconsider my "just-chance" stance on the trend. It also places even more importance on one of my main reasons for being vegetarian for 9 or so years through my youth and into college. Inspired by Diet for a Small Planet, I wanted to be vegetarian because with every step up the food chain, a large amount of energy and resources are wasted (it varies by type of meat, but the general rule of thumb was 10 pounds of vegetable matter and up to 100 gallons of water for each pound of meat, plus a fair amount of pollution of various kinds), and the concentration of toxins and other fat-soluble chemicals (such as PCBs) goes up. Hence, these Arctic populations who were eating fatty Arctic carnivores were about as high as one can go on the food chain and are bearing the brunt of the effects of the poorly-studied chemicals we've been spewing out.
In the Diet for a Small Planet sense of vegetarianism, almost no meat was almost the same as no meat, while my animal-rights-activist friends saw a huge difference between those. Though I don't eat a lot of meat even now, I've been considering going back to having it only once or twice a month, rather than the one to three times a week I've been doing lately. Environmentally, socially, and health-wise, it just makes sense.