Children born in the winter months already have a few strikes against them. Study after study has shown that they test poorly, don't get as far in school, earn less, are less healthy, and don't live as long as children born at other times of year. Researchers have spent years documenting the effect and trying to understand it.

    But economists Kasey Buckles and Daniel Hungerman at the University of Notre Dame may have uncovered an overlooked explanation for why season of birth matters.

    Their discovery challenges the validity of past research and highlights how seemingly safe assumptions economists make may overlook key causes of curious effects. And they came across it by accident.

    In 2007, Mr. Hungerman was doing research on sibling behavior when he noticed that children in the same families tend to be born at the same time of year. Meanwhile, Ms. Buckles was examining the economic factors that lead to multiple births, and coming across what looked like a relationship between mothers' education levels and when children were born.

    'I was just playing around with the data and getting an unexpected result,' Ms. Buckles recalls of the tendency that less educated mothers were having children in winter.

    The two economists, whose offices are across from one another, were comparing notes one day and realized that they might have stumbled across an answer to the season-of-birth puzzle that previous research had overlooked.

    A key assumption of much of that research is that the backgrounds of children born in the winter are the same as the backgrounds of children born at other times of the year. It must be something that happens to those winter-born children that accounts for their faring poorly.

    In a celebrated 1991 paper, economists Joshua Angrist of the Massachusetts Institute of Technology and Alan Krueger of Princeton University argued that season-of-birth differences in how far children go in school is due to how school-attendance laws affect children born at different times of the year. Children born in the winter reach their 16th birthdays earlier in the year than other children, which means they can legally drop out of school sooner in the school year -- which some do, leading to lower education levels in the group.

    There may be validity to all of that research. But if there was any truth to the pattern that Ms. Buckles and Mr. Hungerman discovered, it would question the weightiness of other factors from past research. If winter babies were more likely to come from less-privileged families, it would be natural to expect them to do more poorly in life.

    The two economists examined birth-certificate data from the Centers for Disease Control and Prevention for 52 million children born between 1989 and 2001, which represents virtually all of the births in the U.S. during those years. The same pattern kept turning up: The percentage of children born to unwed mothers, teenage mothers and mothers who hadn't completed high school kept peaking in January every year. Over the 13-year period, for example, 13.2% of January births were to teen mothers, compared with 12% in May -- a small but statistically significant difference, they say.

    'Honestly, when we first saw these patterns, we were so stunned we wondered if we made some mistake,' says Mr. Hungerman. 'We weren't even excited, we were like, 'Is that right?''

    He and Ms. Buckles estimate that family background accounts for up to 50% of the differences in education and earnings. That suggests to them that the compulsory-schooling effect Mr. Angrist and Mr. Krueger described could still be there, but that it can't be used to measure how schooling affects later earnings because it still mixes the effects of privilege and education instead of isolating them.

    冬天出生的孩子可谓先天不足。多项研究表明，他们成绩很差，上学时间较短，薪水较低，健康状况较差，寿命也不如其他季节出生的孩子长。研究人员多年来一直在记录这种效应，并试图了解个中缘由。

    但美国圣母大学（University of Notre Dame）经济学家巴克尔斯（Kasey Buckles）和亨格曼（Daniel Hungerman）可能揭露了出生季节关乎命运的一个被人们忽略的解释。

    他们的发现对以往研究的可靠性提出了挑战，并显示出经济学家做出的看上去很可靠的假定可能忽略了一些奇特效应的关键原因。他们有此发现也是出于偶然。

    亨格曼2007年研究兄弟姐妹的行为时，他注意到一个家庭里的孩子往往都在同样的季节出生。与此同时，巴克尔斯也正在考察导致生育多个孩子的经济因素，他偶然发现母亲的受教育水平和孩子的出生时间之间看上去存在关联。

    巴克尔斯回忆道，我只是在摆弄数据，却得到了意想不到的结果，那就是教育程度较低的母亲往往会在冬天生孩子。

    这两位经济学家的办公室正好相对，某天他们比对各自的研究笔记，意识到他们可能偶然找出了此前的研究一直忽视的出生季节之谜的答案。

    此前大多数这类研究的一个重要假定是，冬天出生的孩子与其他时候出生的孩子具备同样的背景。这些冬天出生的孩子肯定是发生了什么事情所以才命途多舛。

    麻省理工学院（Massachusetts Institute of Technology）经济学家安格瑞斯特（Joshua Angrist）和普林斯顿大学（Princeton University）的克鲁格（Alan Krueger）1991年发表了一篇知名的论文，文中认为，出生季节造成孩子上学时间长短差异的原因在于，义务教育法对不同季节出生的孩子的影响。冬天的孩子早于其他孩子年满16岁，这就意味着他们可以更早地合法离开学校，一些孩子正是这样做的，因此导致这一群体受教育水平较低。

    这可能对所有此类研究都有效。但如果巴克尔斯和亨格曼所发现的模式是真实的，以往研究所提出的其他因素的重要性就值得怀疑。如果冬天出生的孩子更可能来自弱势家庭，那他们命运不如别人也是正常的了。

    上述两位经济学家调查了美国疾病控制与预防中心（Centers for Disease Control and Prevention）提供的1989-2001年之间出生的5,200万人的出生证明数据，这基本上是美国在此期间的所有出生人口。同样的模式不断出现：未婚妈妈、未成年妈妈和教育程度在高中以下的妈妈生育的孩子比例在每年1月达到高峰。举例来说，在这13年中，1月份出生的孩子有13.2%母亲只有十几岁，而5月份出生的孩子这一比例为12%,二者相差不大，但在统计学上具有重要意义。

    亨格曼说，老实说，当我们第一次发觉这种模式时，我们震惊不已，怀疑是不是弄错了。我们甚至一点都不兴奋，而是觉得，这样对吗？

    他和巴克尔斯估计，对于个人间教育程度和收入差异，家庭背景因素所占的比例最多为50%.他们认为，这表明安格瑞斯特和克鲁格所描述的义务教育法的影响可能仍然存在，不过不能用以衡量学校教育对后来的收入有何影响，因为家庭背景和学校教育的影响仍然混杂，而没有分开。