Gabor Szendi:
The gender of offspring
Some want a girl, some want a boy, some have a dream come true and some are disappointed.
To the sorrow of many, the gender of the child is not available on a wish list, and the sex of offspring is determined by evolutionary mechanisms irrespective of parental desires. This does not mean that parents have no say in it, just not in the sense they may want. Many believe it is a coincidence that sperm carrying the Y or X chromosome fertilizes the egg. But just thinking about families where boys or girls are born consecutively, we can already dismiss the idea that the gender of children is dependent merely on chance.
Evolution is driven by the success of reproduction, that is, with whom and to what extent a person is able to spread their genes, that is, how many descendants they will have. Because the male contributes to fertility with minimal investment, while the female carries the common offspring at risk to her health. Therefore according to the parental investment model, a woman has a say in when and from whom to become pregnant, and in the gender of the offspring, depending on environmental and social conditions (Trivers and Willard, 1973). Mating, fertilization and offspring-carrying are a multi-step process, which can be "intervened" in at many points, and women also differ in their ability to assert their evolutionary interests. In addition, this is overruled in favor of evolutionary considerations over those of individuals.
A good example is that in developed countries, 105-106 boys are born to every 100 girls. This is called the secondary gender ratio. (Boy to girl fetus ratio produced during fertilization is the primary ratio). We know that more boys are born because the ratio of men to women between the ages of 20 and 30 is close to 100:100. This shows that processes that appear to be random at the individual level are governed by strict laws at the population level.
"Birth control" in the animal world
Of course, humans are not subject to specific evolutionary rules just for them, so some of the factors influencing gender ratios can also be clearly studied in the animal world. In mammals and birds, we have to assume that females always reproduce, while males reproduce only when they can fight against other males for reproduction rights. Thus, the female must "consider" whether she can give birth to a large, dominant male offspring who will in turn have many offspring, or be sure to give birth to a female offspring. When food is low during the breeding season, it is a better idea to give birth to a female because she has less stature and eats less, and is more likely to reach reproductive age, while the malnourished male offspring, if he grows up, will become too weak to fight successfully with other males.
In farm-raised deer, 75% of the offspring were male when the oil content of their food was increased, whereas only 46% male were born when the calorific content was reduced. Even more dramatic differences were found in mice, with three times as many female mice as male being born on low-fat foods (Rosenfeld and Roberts, 2004). In wild horses, it has been found that of 3% of females with impaired physical condition, 46% of those who maintain their condition and 80% of those in improved condition give birth to a male offspring (Cameron and Linklater, 2007). Female opossums, if their diet is enriched with sardines, produce more male offspring (Rosenfeld and Roberts, 2004).
African women were also observed to have a 100:100 gender ratio for the offspring of malnourished, small females, while well-nourished, taller women gave birth to 125 boys to every 100 girls (Andersson and Bergström, 1998). In an English study, 56% of women who consumed the most calories before fertilization, and 45% of those who had the fewest calories gave birth to a boy (Mathews et al., 2008).
The dominant mother
The fact that a female gets more food than others, despite her limited capabilities, suggests that she is stronger, more pushy, and more dominant. This beneficial property is associated with fetal and subsequent testosterone levels in both men and women. In red deer it has been observed that the more dominant females give birth to more male offspring (Clutton-Brock et al, 1984). Numerous studies have confirmed that more dominant macaque females give birth to more sons (Schino et al., 1999). However instead of giving further animal examples, we are more interested in whether this relationship is true in man as well.
According to Valerie Grant, author of the dominant mother theory, a woman's dominant personality in modern society is demonstrated by her success. Therefore, the gender rate of children of successful women in different fields was collected from two biographical lexicons. In one volume, the male-to-female ratio of the dominant female is 139:100, in the second it was 143:100, far exceeding the 106:100 ratio (Grant and Yang, 2003). High levels of fetal testosterone slows down the development of the left hemisphere of the brain, which can lead to all kinds of learning disabilities and delayed speech development. Testosterone theory is reinforced by the fact that women with such problems give birth to three times as many boys as girls (Tallal et al., 1989).
I have previously written about testosterone levels and index finger to ring finger ratios. In short, a longer ring finger in both genders indicates greater fetal testosterone activity. For women, the smaller the 2:4 finger ratio (that is, the longer the ring finger), the more dominant the personality, and the more likely they are to give birth to a boy (Kim et al., 2013). In a study of women with 'masculine' occupations, a ratio of 130:100 boys to girls was found (Bernstein, 1954).
Certain occupations seriously affect the gender ratio of offspring in an unexplained way. The ratio is 59:100 for fighter pilots, 47:100 for helicopter pilots, and 72:100 for civilian pilots active in civil aviation. For divers, it is 53:100 (Grant and Metcalf, 2003). For some reason, male anesthetists also have an increased proportion of girls (Gupta et al., 2013). For runners, the ratio of boys to girls decreases in proportion to the increasing weekly mileage, but when a run is just missed for a while, the number of boys doubles compared to girls (Crawford et al., 1992).
What about men?
It does not seem logical that the biological characteristics of males should not influence the gender of offspring, since the male offspring of a dominant male has a good chance of inheriting the characteristics of his father, that is, it is in the mother's best interest to give birth to a son. High testosterone levels make men aggressive, and its extreme form is the abusive husband. Abusive males have 12% more sons than more gentle fathers (Kanazawa, 2006). Men also follow the rule that the smaller the 2: 4 finger ratio, the more likely the offspring will be boy (Manning et al., 2002).
Because height is strongly influenced by testosterone levels, it has also been shown that the taller a man is, the more likely he is to have male children (Kanazawa, 2005). Height and dominance are important in politics too, with the tallest candidate winning in American presidential elections, and the gender ratio of children of American presidents is 146:100 (Coney and Mackey, 1998). Perhaps it is also related to dominance, or perhaps well-being, that in royal families the boy-to-girl ratio is 137:100 (Bernstein, 1948). Or it depends on the lucky choice you see in the case of the pretty boy-looking, late Diana Princess of Wales, or Kate Middleton, Duchess of Cambridge, where both their height and Kate's finger ratio indicate more serious testosterone effects.
Evolution never "thinks" about an individual, but about a population. At the individual level, we can identify the direct factors that influence the gender of offspring. At the population level, however, the relative stability of the gender ratio is a much more mysterious phenomenon. How do individuals know whether they need to produce a boy or girl child to achieve a sex ratio of 106:100 at the population level? A Finnish study from 1775 to 1850 analyzed the sex ratio of births and found that as the proportion of men in the population decreased, the number of male births began to increase (Lummaa et al., 1998).
Other studies have shown that after wars, the high male deficiency rate is immediately followed by an increased male birth rate (Ellis and Bonin, 2004). It is assumed that women are somehow able to compile statistics on the gender ratio of their environment, thereby increasing the likelihood of having a child of the more popular gender. Of course, this will eventually lead to "overproduction" of the gender which is depleted, therefore, the gender ratio changes cyclically (James, 1998).
The effect of stress
The stress experienced by the mother affects not only fertility but also the gender of the offspring, because stress "tells" what can be expected in the future, whether it is worth giving birth, and if so, what kind of child is more likely to be successful in the process of reproduction.
Animals experience that stressors, for example, lack of food or overcrowding, reduce the birth gender at the expense of males. When medically inhibiting the production of stress hormones, normal male-to-female ratios were restored under stressful conditions (Rosenfeld and Roberts, 2004). Acute and chronic stress have different effects on gender at birth. Unexpected disaster, for example, an earthquake (Fukuda et al., 1998), a short war (Zorn et al., 2002), or personal tragedies reduce the birth rate of boys. However, prolonged stress increases women's testosterone levels (Grant, 2007), thus increasing the proportion of boys. This is one possible explanation for the increasing birth rate of boys during and after wars. At the same time, women exposed to long-term working stress have an increased girl birth rate (Ruckstuhl et al., 2010).
Many people may think that it would be good to have some influence on the gender of the unborn child. However, it may not be such a good thing to interfere with nature. Time and time again, nature has proven itself to be wiser than us.
References
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