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母乳喂养也会“重男轻女”?

更新时间:2017-1-24 18:52:52 来源:纽约时报中文网 作者:佚名

Does Breast Milk Have a Sex Bias?
母乳喂养也会“重男轻女”?

In 1973, the biologist Robert Trivers and the computer scientist Dan Willard made a striking prediction about parents and their offspring. According to the principles of evolutionary theory, they argued, the male-to-female ratio of offspring should not be 50-50 (as chance would dictate), but rather should vary as a function of how good (or bad) the conditions are in which the parents find themselves.

1973年,生物学家罗伯特·特里弗斯(Robert Trivers)和计算机学家丹·威拉德(Dan Willard)做出了一个关于父母与子女的惊人预言。他们表示,按照进化论的原则,子女的男女比例应为50:50(从概率上说是如此),但这一比例其实应是父母境况的函数,会依父母境况的好坏程度而变。

Are the parents’ resources plentiful — or scarce? The Trivers-Willard hypothesis holds that when their conditions are good, parents will have more male offspring: Males with more resources are likely to gain access to more females, thereby increasing the frequency with which their genes (and thus their parents’ genes) are preserved in future generations. Conversely, male offspring that lack resources are likely to lose out to males that have more resources, so in bad conditions it pays for parents to “invest” more in daughters, which will have more opportunities to mate.

父母拥有的资源是丰富——还是匮乏呢?“特里弗斯-威拉德假设”认为,如果父母的境况很好,那他们就会生下更多儿子:占有更多资源的男性可能有机会接触更多女性,如此一来,其基因(乃至其父母的基因)由下一代延续下去的机会也就更大。反之,资源匮乏的男性后代可能竞争不过资源丰富的男性,如此一来,要是境况不佳,父母“投资”于女儿就会更划算,因为后者日后在婚配市场上有更多机会。

It follows, as a kind of corollary, that when parents have plentiful resources they will devote those resources more to their sons, whereas when resources are scarce, parents will devote them more to their daughters.

于是,作为一种必然的结果,当父母占有丰富的资源时,他们会把这些资源更多地投入到儿子身上;而当资源较为匮乏时,则会将资源更多地投入到女儿身上。

In short: If things are good, you have more boys, and give them more stuff. If things are bad, you have more girls, and give more of your stuff to them.

简而言之:如果条件好,你会生下更多男孩,并把资源更多地分配给他们。如果条件不好,你会生下更多女孩,并把自己的资源更多地分配给她们。

Is this hypothesis correct? In new research of ours, to be published in the April issue of The Journal of Experimental Child Psychology, we suggest that in the case of breast-feeding, at least, it appears to be.

这个假设是正确的吗?我们在一项新研究中发现,至少在母乳喂养的情况下看起来的确如此。该研究的成果将被发表在《实验儿童心理学杂志》(The Journal of Experimental Child Psychology) 4月号上。

In recent years, evidence has emerged suggesting that in various mammalian species, breast milk — which is, of course, a resource that can be given to children — is tailored for the sex of each offspring. For example, macaque monkey mothers produce richer milk (with higher gross energy and fat content) for sons than for daughters, but also provide greater quantities of milk and higher concentrations of calcium for daughters than for sons.

近年来已经有一些证据表明,在各种哺乳动物中,母乳——这当然也算一种可以被给予子女的资源——是针对每个孩子的性别定制的。譬如,比起为女儿分泌的乳汁,猕猴母亲为雄性猴宝宝分泌的乳汁营养更丰富(所蕴含的总能量和脂肪量都更高);相较之下,为雌性猴宝宝分泌的数量更大,钙含量也更高。

There is also some evidence of sex-biased milk production among human mothers. A group of women studied in Massachusetts, for example, produced higher-quality milk, with greater energy, lipids and other constituents, for their sons than for their daughters. Economically sufficient Kenyan mothers, according to another study, produced milk with higher fat concentration for their sons than for their daughters, whereas the reverse was true for poorer mothers.

还有证据显示,人类母亲的乳汁分泌也存在基于孩子性别的差异。举个例子,马萨诸塞州的一组女性受试者为儿子分泌的乳汁比为女儿分泌的质量更高,含有更多能量、脂质以及其他营养成分。根据另一项研究,富足的肯尼亚母亲为儿子分泌的乳汁比为女儿分泌的含有更多脂肪;如果母亲较为贫穷,情况则恰恰相反。

The aim of our research was to figure out how to study the divergent effects of breast milk on sons and daughters on a much larger scale. Our solution: to look at twins.

我们这项研究旨在弄清如何就母乳对儿子和女儿的不同影响进行更大规模的研究。我们的办法是:观察双胞胎。

In light of the emerging evidence of sex-specific variation in breast milk quality, the Trivers-Willard hypothesis implies that breast-feeding may be less beneficial for opposite-sex twins than for same-sex twins. Same-sex twins, after all, can benefit from their mother’s sex-tailored breast milk just as non-twins can. However, the breast milk of mothers of opposite-sex twins cannot be tailored for either sex. Perhaps it is selectively tailored for the wrong sex part of the time, or for neither sex all of the time.

鉴于母乳质量因孩子性别不同而存在差异的证据不断浮出水面,特里弗斯-威拉德假设意味着母乳给龙凤胎带来的好处要少于给同性双胞胎带来的好处。毕竟,同性双胞胎从母亲依照孩子的性别定制的乳汁中享有的好处,与非双胞胎没有差别。但母亲的乳汁不可能同时为龙凤胎中的两个人定制。它有可能在某个时间段针对错误的性别被定制出来,还有可能一直都没针对任何一种性别被定制。

Thus, the Trivers-Willard hypothesis led us to surmise that opposite-sex twins would be disadvantaged for health and growth. This would be reflected in their being shorter and weighing less than same-sex twins.

因此,特里弗斯-威拉德假设引得我们猜测,龙凤胎在健康和生长方面会处于劣势。相关表现则会是,与同性双胞胎相比,他们的身高更矮,体重更轻。

Using data from the National Longitudinal Study of Adolescent Health — a large, nationally representative, long-term behavioral and medical study of young Americans — we tested this idea. More than 20,000 adolescents were interviewed in their homes several times: once in 1994 or 1995, and again in 1996, 2001 to 2002, and 2007 to 2008. The subjects’ self-reported height and weight were recorded at all four points, and their height and weight were also directly measured by interviewers from 2007 to 2008, yielding five data points. Information about breast-feeding was provided by mothers.

我们利用来自国家青少年健康纵向研究(US National Longitudinal Study of Adolescent Health)的数据对上述想法进行了检验。在这项针对美国年轻人的大型、长期、全国性行为和医学研究中,曾有超过2万名青少年在家中数次接受调查:一次是在1994或者1995年,然后是在1996年,2001至2002年,以及2007至2008年。被调查者每次自行报告的身高和体重均被记录在案,形成了四个点,调查者还在2007至2008年直接测量了他们的身高和体重,从而形成了第五个数据点。关于母乳的信息则由母亲提供。

Our study focused on 546 same-sex individual twins (277 females and 269 males) and 233 opposite-sex individual twins (120 females and 113 males). Using all five measures of height and weight, we compared their growth over time.

我们的研究所聚焦的对象,有546人属于同性双胞胎(277名女性和269名男性),有233人属于龙凤胎(120名女性和113名男性)。我们借助关于其身高和体重的五次测量结果,对比了他们的长期生长情况。

We found that breast-fed same-sex twins were indeed either slightly taller or substantially taller than breast-fed opposite-sex twins at four of the five time points (the exception was the first, when they were just entering adolescence). Same-sex twins were, on average, nearly one inch taller than their opposite-sex counterparts. Similarly, same-sex twins were substantially heavier than the opposite-sex twins except during the first measurement period. The same-sex twins were, on average, 12 pounds heavier than the opposite-sex twins.

我们发现,其中四次测量结果显示,比起以母乳哺育的龙凤胎,以母乳哺育的同性双胞胎的身高的确稍高一点,甚至高出很多(第一次是例外,那时候他们刚刚进入青春期。)同性双胞胎比龙凤胎平均高出将近一英寸。同样地,比起龙凤胎,同性双胞胎的体重重了不少,只有第一次测量的结果是例外。同性双胞胎比龙凤胎平均重12磅。

To the best of our knowledge, ours is the first study to demonstrate that there are observable physical consequences of sex bias in human breast milk. Additional research is needed, of course, to confirm the presence of sex bias in human breast milk and the differences in the quality of breast milk in accordance with the conditions of the parents. But for now, the Trivers-Willard hypothesis seems to hold.

据我们所知,我们的研究首次证明,人类母乳基于孩子性别的差异,能够带来具有可观测性的生理后果。当然了,还得进行额外的研究,以确认由于父母的境况不同,人类乳汁的确存在基于孩子性别的差异,乳汁的质量的确会有所不同。但就目前而言,特里弗斯-威拉德假设似乎站得住脚。

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