A Test of the Trivers-Willard Hypothesis with Three Samples of High-achieving Men: a Tenuous Fit for an Altricial, Fathering-intensive Species

Abstract

The Trivers-Willard hypothesis argues for a facultative (secondary) sex ratio in sexually reproducing species. To wit: Given that males have a potentially greater variance in numbers of offspring than do females, mothers would bias toward sons if the rearing environment for the son would be superior to that of competitors and, thus, give the son an added ply of attractiveness to females. Conversely, mothers would bias toward daughters if the rearing environment of the daughter would be inferior to that of competitors. A highly attractive daughter portends many grandchildren, but not as many as her equally attractive brother, and fewer grandchildren, but not as few as her unattractive brother, if she is unattractive. Data from three samples of high-achieving men offer more support for Fisher's basic theory than for the Trivers-Willard hypothesis. It is suggested that a highly altricial and fathering-intensive species, e.g. Homo sapiens, would render the Trivers-Willard hypothesis problematic. Key Words: Trivers-Willard hypothesis; Fisher; Facultative (secondary) sex ratio; Social father. The great tragedy of science the slaying of a beautiful hypothesis by an ugly fact - Thomas Huxley Tis a happy thing to be the father unto many sons - William Shakespeare (Henry VI) The gender of an organism is an important, if not decisive, predictor of that organism's life-chances and, thereby, life-trajectory. Humans are neither an exception nor transcendent. This article examines two ideas which provide a context to the sex ratio at birth (secondary sex ratio) . Facultative Sex Ratio: Fisher Fisher (1930) developed a theory stating that the gender of an individual in a sexually reproducing species would not follow a coin flip analogy across the population /specie s. In essence, total parenting energies in rearing their young would be equally divided between the two genders. Thus, if an individual in one gender - e.g. female - required, on average, more energy to rear than an individual in the alternate gender - e.g. male - then a greater number of the less costly gender would be born (if not conceived, primary sex ratio). Accordingly, the total energy of rearing the two genders would reach equilibrium at birth. Given that males, as a class, are more vulnerable to lethal agents prior to puberty, a cohort of males would require less energy than a cohort of their less vulnerable sisters. Hence, we would expect that more sons are born than daughters, and such is the case. Across the world's community of populations, the secondary sex ratio is approximately 105 sons born per 100 daughters (Central Intelligence Agency 2008). ' This biased ratio is independent of virtually any plausible variable. For the U.S., the reported sex ratio at birth has held steady for decades (U.S. Bureau of the Census, 2008). Facultative Sex Ratio: Trivers-Willard Trivers & Willard (1973) refined this core idea with the following model: Given that (1) males generally invest less time and resources in the rearing of their offspring than do females, and that (2) males can be much more prolific than females, i.e. the variance of male reproductive success can be greater than that of the female, then a route to large numbers of descendants by a parent would be to rear sons who would be highly attractive to many females. However, a son is somewhat of a gamble. If he is unwaveringly unattractive to females, the son may well remain chaste and celibate throughout his life with no descendants for his parents at all. Compared to their brothers, females are more limited in the potential numbers of children they can procreate. But daughters are less of a gamble. Even unattractive females have little trouble finding suitors who will mate with them. Hence, a daughter is more of a guarantee of some descendants, but is less capable of generating as large a number of descendants as is her brother: her mother's son. …