Anderson School of Management, University of California Los Angeles, Los Angeles, California, USA.
Program in Medical and Population Genetics, Broad Institute of MIT and Harvard, Cambridge, Massachusetts, USA.
Am J Hum Biol. 2023 Jul;35(7):e23887. doi: 10.1002/ajhb.23887. Epub 2023 Mar 2.
This study explores the magnitude and timing of sex and gender disparities in child development by describing differences in health outcomes for male and female siblings, comparing twins to control for all aspects of life circumstances other than sex and gender.
We construct a repeat cross-sectional dataset of 191 838 twins among 1.7 million births recorded in 214 nationally representative household surveys for 72 countries between 1990 and 2016. To test for biological or social mechanisms that might favor the health of male or female infants, we describe differences in birthweights, attained heights, weights, and survival to distinguish gestational health from care practices after each child is born.
We find that male fetuses grow at the expense of their co-twin, significantly reducing their sibling's birthweight and survival probabilities, but only if the other fetus is male. Female fetuses are born significantly heavier when they share the uterus with a male co-twin and have no significant difference in survival probability whether they happen to draw a male or a female co-twin. These findings demonstrate that sex-specific sibling rivalry and male frailty begin in utero, prior to gender bias after birth that typically favors male children.
Sex differences in child health may have competing effects with gender bias that occurs during childhood. Worse health outcomes for males with a male co-twin could be linked to hormone levels or male frailty, and could lead to underestimates of the effect sizes of later gender bias against girls. Gender bias favoring surviving male children may explain the lack of differences in height and weight observed for twins with either male or female co-twins.
本研究通过描述男性和女性兄弟姐妹在健康结果方面的差异，比较同卵双胞胎和对照组，以控制除性别以外的所有生活环境因素，探讨儿童发育中的性别差异的程度和时间。
我们构建了一个重复的横断面数据集，其中包括 191838 对双胞胎，来自 1990 年至 2016 年期间在 72 个国家的 214 个具有代表性的家庭调查中记录的 170 万例出生。为了测试可能有利于男性或女性婴儿健康的生物学或社会机制，我们描述了出生体重、达到的身高、体重和存活率的差异，以区分胎儿期健康和每个孩子出生后的护理实践。
我们发现，男性胎儿以牺牲其同卵双胞胎为代价生长，显著降低了他们兄弟姐妹的出生体重和存活概率，但前提是另一个胎儿也是男性。当女性胎儿与男性同卵双胞胎共享子宫时，她们出生时的体重明显更重，无论她们碰巧与男性还是女性同卵双胞胎一起出生，她们的存活概率都没有显著差异。这些发现表明，胎儿期存在特定于性别的同胞竞争和男性脆弱性，这发生在出生后的性别偏见之前，而这种性别偏见通常有利于男性儿童。
儿童健康方面的性别差异可能与儿童期发生的性别偏见存在相互竞争的影响。与男性同卵双胞胎的男性同胞健康状况较差可能与激素水平或男性脆弱性有关，并可能导致对后来针对女孩的性别偏见的影响大小的低估。有利于幸存的男性儿童的性别偏见可能解释了在同卵双胞胎中无论是男性还是女性同卵双胞胎，身高和体重都没有差异的原因。
