Twins Less Frequent Than Expected Among Male Births in Risk Averse Populations

Abstract

Male twin gestations exhibit higher incidence of fetal morbidity and mortality than singleton gestations. From an evolutionary perspective, the relatively high rates of infant and child mortality among male twins born into threatening environments reduce the fitness of these gestations, making them more vulnerable to fetal loss. Women do not perceive choosing to spontaneously abort gestations although the outcome may result from estimates, made without awareness, of the risks of continuing a pregnancy. Here, we examine whether the non-conscious decisional biology of gestation can be linked to conscious risk aversion. We test this speculation by measuring the association between household surveys in Sweden that gauge financial risk aversion in the population and the frequency of twins among live male births. We used time-series regression methods to estimate our suspected associations and Box-Jenkins modeling to ensure that autocorrelation did not confound the estimation or reduce its efficiency. We found, consistent with theory, that financial risk aversion in the population correlates inversely with the odds of a twin among Swedish males born two months later. The odds of a twin among males fell by approximately 3.5% two months after unexpectedly great risk aversion in the population. This work implies that shocks that affect population risk aversion carry implications for fetal loss in vulnerable twin pregnancies.

Twinning in Norway Following the Oslo Massacre: Evidence of a ‘Bruce Effect’ in Humans

“…Applying the three steps in our main test described above to the natural logarithm of the odds of a male among live singleton births yielded the results shown in Table 3. We found, consistent with the Bruce Effect and with timing reported in other literature (Karasek et al, 2015), that the secondary sex ratio dropped significantly 5 months after the Oslo Massacre.…”

“…In non-human animals, the Bruce Effect may provide a female counterstrategy to infanticide, or may be an adaptive strategy to limit investment in gestations that face a high risk of death (Labov, 1981). Mechanisms associated with the Bruce Effect likely include the endocrine stress response (Beehner et al, 2005;Cheney & Seyfarth, 2009), suggesting that the Bruce Effect may be part of a generalized female reproductive response to environments that threaten offspring.Theory (Haig, 1999;Schooling, 2014;Stearns, 1987;Trivers & Willard, 1973;Wells, 2000) and empirical work in human populations (Bruckner et al, 2015;Karasek et al, 2015;Orzack et al, 2015) suggest that natural selection has conserved endemic selection in utero that allows women to spontaneously abort gestations least likely to yield grandchildren. Acute stressors on a population appear, moreover,…”

“…Theory (Haig, 1999; Schooling, 2014; Stearns, 1987; Trivers & Willard, 1973; Wells, 2000) and empirical work in human populations (Bruckner et al, 2015; Karasek et al, 2015; Orzack et al, 2015) suggest that natural selection has conserved endemic selection in utero that allows women to spontaneously abort gestations least likely to yield grandchildren. Acute stressors on a population appear, moreover, to induce epidemic selection in utero via the maternal stress response that reportedly raises the level of fetal fitness required for a gestation to continue (Catalano & Bruckner, 2006; Catalano et al, 2008; 2009; 2015; Navara, 2014).…”

Reproductive suppression, birth defects, and periviable birth

“…We acknowledge that invoking the construct of gestational loss aversion, despite its intuitive appeal, raises issues we cannot address with our data. Other literature (Karasek et al., ) has, for example, asked whether women who exhibit gestational loss aversion also exhibit loss aversion in investment choices they can describe making. Research suggests they may because the male twin ratio of birth cohorts, which measures the spontaneous abortion of the smaller of male twin fetuses, falls in Sweden when households report rising economic risk aversion (Karasek et al., ).…”

“…Other literature (Karasek et al., ) has, for example, asked whether women who exhibit gestational loss aversion also exhibit loss aversion in investment choices they can describe making. Research suggests they may because the male twin ratio of birth cohorts, which measures the spontaneous abortion of the smaller of male twin fetuses, falls in Sweden when households report rising economic risk aversion (Karasek et al., ). The literature also reports that Danish conception cohorts that yield more than expected spontaneous abortions also yield more than expected elective abortion of gestations with no clinical indication of fetal defect (Catalano, Bruckner, et al., ).…”