An adaptive significance of morning sickness? Trivers-Willard and Hyperemesis Gravidarum.

Nausea during pregnancy, with or without vomiting, is a common early indication of pregnancy in humans. The severe form, Hyperemesis Gravidarum (HG), can be fatal. The aetiology of HG is unknown. We propose that HG may be a proximate mechanism for the Trivers-Willard (T-W) evolutionary hypothesis that mothers in poor condition should favor daughters. Using Swedish linked registry data, 1987-2005, we analyze all pregnancies that resulted in an HG admission and/or a live birth, 1.65 million pregnancies in all. Consistent with the T-W hypothesis, we find that: (i) HG is associated with poor maternal condition as proxied by low education; (ii) HG in the first two months of pregnancy is associated with a 7% point increase in live girl births; and (iii) HG affected pregnancies have a 34-percent average rate of inferred pregnancy loss, higher among less educated women.

Effect of Registered Partnership on Labor Earnings and Fertility for Same-Sex Couples: Evidence From Swedish Register Data.

The expansion of legal rights to same-sex couples is a foot in a number of Western countries. The effects of this rollout are not only important in their own right but can also provide a window on the institution of marriage and the rights bundled therein. In this article, using Swedish longitudinal register data covering 1994-2007, we study the impact of the extension of rights to same-sex couples on labor earnings and fertility. In 1994, registered partnership for same-sex couples was introduced, which conferred almost all rights and obligations of marriage--a notable exception being joint legal parenting, by default or election. The latter was added in the 2002 adoption act. We find registered partnership to be important to both gays and lesbians but for different reasons. For gays, resource pooling emerges as the main function of registered partnerships. For lesbians, registered partnership appears to be an important vehicle for family formation, especially after the 2002 adoption act. In contrast to heterosexual couples (included for comparison), we find no evidence of household specialization among lesbians. The lack of specialization is noteworthy given similar fertility effects of registered partnership (after 2002) and the fact that lesbian couples were less assortatively matched (on education) than heterosexual couples--children and unequal earnings power being two factors commonly believed to promote specialization.

Girl adoption in China-A less-known side of son preference.

In 1987, 4 per cent of girls were adopted within China. Why? Unlike infanticide, abandonment rids parents of daughters while preserving the supply of potential brides. In fact, an erstwhile tradition common in Fujian and Jiangxi provinces had parents of sons adopting an infant girl to serve as a future daughter-in-law and household help. Analysing a nationally representative 1992 survey of children, we found that: (1) girl adoptions were concentrated in the above-mentioned provinces; (2) girls were predominantly adopted by families with sons; (3) adopted girls faced substantial disadvantage as measured by school attendance at ages 8-13. In the 1990s, as the sex ratio at birth climbed, were girls aborted rather than abandoned? Observing that in the 2000 census too many girls appear in families with older sons, we estimated that at least 1/25 girls were abandoned in the 1990s, a proportion that in Fujian and Jiangxi may have peaked at 1/10 in 1994.

Son-biased sex ratios in the 2000 United States Census.

We document male-biased sex ratios among U.S.-born children of Chinese, Korean, and Asian Indian parents in the 2000 U.S. Census. This male bias is particularly evident for third children: If there was no previous son, sons outnumbered daughters by 50%. By contrast, the sex ratios of eldest and younger children with an older brother were both within the range of the biologically normal, as were White offspring sex ratios (irrespective of the elder siblings' sex). We interpret the found deviation in favor of sons to be evidence of sex selection, most likely at the prenatal stage.

Trivers-Willard at birth and one year: evidence from US natality data 1983-2001.

Trivers & Willard (TW) hypothesized that evolution would favour deviations from the population sex ratio in response to parental condition: parents in good condition would have more sons and parents in poor condition would have more daughters. We analyse the universe of US linked births and infant deaths to white mothers 1983-2001, covering 48 million births and 310,000 deaths. We find that (i) married, better educated and younger mothers bore more sons and (ii) infant deaths were more male if the mother was unmarried and young. Our findings highlight the potential role of offspring sex ratio as an indicator of maternal status, and the role of infant mortality in shaping a TW pattern in the breeding population.

Hermaphroditism: What's not to like?

Hermaphroditism is rare and phylogenically in decline among animal species. The evolutionary basis for this development is not well understood. This paper focusses on self-incompatible simultaneous hermaphroditism in animals. It proposes that such hermaphroditism is not stable in sufficiently heterogeneous populations, suggesting a possible reason for why hermaphroditism is rare among evolved animal species. The argument turns on the Bateman principle, namely that male reproductive success (RS) is limited by partner availability, while female RS is not. We show that: low-quality individuals do better if female; secondary sexual differentiation may be important for understanding the existence of males; and that hermaphroditic mating is reciprocal. Reciprocity may be key to understanding promiscuity and attendant phenomena such as cryptic female choice, sperm competition and love darts-common features of hermaphroditic mating. We also argue that hermaphrodites are especially vulnerable to male violence, suggesting a reason for the rarity of trioecy. Finally, we propose that external fertilization, and the scope for streaking, may be one reason fish are the only simultaneously hermaphroditic vertebrates.