Update 12-31-17 Unfortunately many of the links to studies are no longer working but I read all of them and have deleted the broken links. The data is real, the studies were done, it's just that the links are not working any more.
The idea that males are more fragile than females and therefore more of them are conceived because more are lost, is at the root of much of gender swaying. 140-160 males are conceived for every 100 females, falling to 106 boys born for every 100 girls.
Scientists have known for some time that rates of conception of children of BOTH genders seem to vary by season and that this phenomenon varies depending on location. Changes in the seasonal distribution of births in Görlitz, Germany, during the period between 1657 and 1816. - PubMed - NCBI Previous studies have indicated that in Western Europe, October is the easiest month for conceiving, and April is the most difficult. From an evolutionary perspective, this makes sense because babies conceived in the fall months are born in summertime when the livin' is easy, whereas babies conceived in spring are born in the darkest months of winter and in the native environment were probably less likely to survive. For the good of the species, some mechanism that encourages more babies to be born when food is plentiful and they have a greater chance of survival, has apparently evolved at some point.
Dr. Angelo Cagnacci, an Italian researcher at the Policlinico of Modena (and his research team) set out to investigate this phenomenon and discovered something really amazing. In a study of over 14,000 births, in the Modena, Italy area, not only were conception rates lowest in March-May and highest from Sept.-November as previous research had indicated, but the gender ratio shifted during those periods as well.
More girls were conceived in the low-conception months of March-May, peaking in April (the ratio was 487 boys to 513 girls) More boys were conceived in the high-conception months of Sept-Nov, peaking in October. (535 boys and 465 girls). Keeping in mind that virtually everything that reduces fertility seems to sway pink and virtually everything that increases fertility seems to sway blue, it would appear that there is some sort of natural seasonal variation in fertility that also acts to sway in some fashion.
Why, then, are any boys born in the wintertime at all? Imagine if you will, a sort of continuum of fertility. All of us fall somewhere along this continuum, some of us more fertile and some of us less fertile; some get pregnant just by walking past their husand in the hallway and others have to try for many months in order to conceive and then everyone else falls somewhere in between. Highly fertile couples can probably conceive a baby boy to be born in wintertime with no problem at all, whereas less fertile couples may only be able to conceive a boy to be born in summertime (of course there are SO MANY variables at play that this would just be an overall tendency and obviously there would be MANY exceptions, this description is just meant to help envision what the real-world applications of such a mechanism might be)
Using this information in swaying is a bit tricky, because the evidence indicates that where you live on Planet Earth affects this phenomenon. For reasons that we do not yet comprehend, more girls than would be statistically expected are born at the equator, and more boys at northern latitudes. Mother Nature seems to "time" conceptions in parts of the world that experience winter, so more male babies will be born in optimal conditions. If you live around the equator, this information may not apply to you.
Anyway, according to Cagnacci, the best conditions for conceiving a baby of ANY gender (and therefore, a boy) are a day length of 12 hours and an average temperature of 12°C. Maximum conception rates take place at opposite times in regions on either side of the equator. In the Southern Hemisphere (even though this has not been studied) it is predicted that the opposite trend would be true. Fewer babies (but more girls) should be conceived in Sept-November in the Southern Hemisphere, and more babies (and more boys) in March-May.
(BTW this is NOT what is predicted by the ion theory that is part of traditional swaying lore. The ion theory claims that winter conceptions should = more boys and summer conceptions should = more girls, and that spring and fall are neutral.)
Why might this be? Babies conceived Sept - Nov. will be born in the summer months when the climate is pleasant and food abundant, and babies that are conceived March - May will be born Dec-Feb, when temps are coldest and food is scarcest. Baby boys need more calories than baby girls throughout pregnancy and after birth, and in fact throughout LIFE. http://www.bmj.com/content/326/7401/1245.full.pdf (higher caloric needs during boy pregnancy) How Many Calories Does Your Child Need? (males have higher caloric needs throughout infancy and childhood)
As an example, the estimated average requirement for baby boys for their first 6 months of life is 545-690 calories per day, whereas baby girls require less, on average 515-645 kcal per day. It is not hard to imagine that it is easier for mothers to provide more nutrition via breastfeeding, to baby boys during times when food is ample - on a personal note, I often find I cannot even produce enough breastmilk without drinking three Dr. Peppers a day (at 150 calories a pop) in addition to eating a balanced diet, so it makes a lot of sense to me on a gut level that such a mechanism might evolve over millions of years. It's not only in the first few months that those early calories matter - if a baby boy is chronically undernourished during his first few months of life, even if he survives infancy he may be at a disadvantage for the rest of his life, and therefore be less likely to survive and reproduce.
Also, there may be another strange mechanism at play. Three studies I have found Ambient temperature predicts sex ratios and male longevity (here is a longer version of that same study http://precedings.nature.com/documen...20093915-1.pdf), http://www.springerlink.com/content/5n7u45gmlqwpc88x/ (abstract only), and http://www.ncbi.nlm.nih.gov/pubmed/16840640 (somehow I have the full text of this article in my files but I can only find the abstract now, sorry!) both seem to indicate that cold temperatures at time of conception and in early pregnancy seem to mean more females conceived/born and warmer temperatures, even only just one degree warmer, raised the odds that a male would be conceived/born.
(Again, this completely flies in the face of the ion theory. According to the ion theory, boys "like" cold things and girls "like" warm things.)
However, this is confuddling because of the study that found more girls were born in tropical latitudes. I ~personally~ believe that this may have more to do with other factors such as diet and cultural issues - in fact, according to one study, Africa overall had more girls than would be statistically expected, even the nations that are far from the equator such (such as South Africa - sex ratio of 102 boys for 100 girls) and some equatorial nations had exactly as many boys as would be statistically expected (Nigeria 106 boys for 100 girls). http://en.wikipedia.org/wiki/List_of...s_by_sex_ratio
