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.

    冬天出生的孩子可謂先天不足。多項(xiàng)研究表明，他們成績(jī)很差，上學(xué)時(shí)間較短，薪水較低，健康狀況較差，壽命也不如其他季節(jié)出生的孩子長(zhǎng)。研究人員多年來一直在記錄這種效應(yīng)，并試圖了解個(gè)中緣由。

    但美國(guó)圣母大學(xué)（University of Notre Dame）經(jīng)濟(jì)學(xué)家巴克爾斯（Kasey Buckles）和亨格曼（Daniel Hungerman）可能揭露了出生季節(jié)關(guān)乎命運(yùn)的一個(gè)被人們忽略的解釋。

    他們的發(fā)現(xiàn)對(duì)以往研究的可靠性提出了挑戰(zhàn)，并顯示出經(jīng)濟(jì)學(xué)家做出的看上去很可靠的假定可能忽略了一些奇特效應(yīng)的關(guān)鍵原因。他們有此發(fā)現(xiàn)也是出于偶然。

    亨格曼2007年研究兄弟姐妹的行為時(shí)，他注意到一個(gè)家庭里的孩子往往都在同樣的季節(jié)出生。與此同時(shí)，巴克爾斯也正在考察導(dǎo)致生育多個(gè)孩子的經(jīng)濟(jì)因素，他偶然發(fā)現(xiàn)母親的受教育水平和孩子的出生時(shí)間之間看上去存在關(guān)聯(lián)。

    巴克爾斯回憶道，我只是在擺弄數(shù)據(jù)，卻得到了意想不到的結(jié)果，那就是教育程度較低的母親往往會(huì)在冬天生孩子。

    這兩位經(jīng)濟(jì)學(xué)家的辦公室正好相對(duì)，某天他們比對(duì)各自的研究筆記，意識(shí)到他們可能偶然找出了此前的研究一直忽視的出生季節(jié)之謎的答案。

    此前大多數(shù)這類研究的一個(gè)重要假定是，冬天出生的孩子與其他時(shí)候出生的孩子具備同樣的背景。這些冬天出生的孩子肯定是發(fā)生了什么事情所以才命途多舛。

    麻省理工學(xué)院（Massachusetts Institute of Technology）經(jīng)濟(jì)學(xué)家安格瑞斯特（Joshua Angrist）和普林斯頓大學(xué)（Princeton University）的克魯格（Alan Krueger）1991年發(fā)表了一篇知名的論文，文中認(rèn)為，出生季節(jié)造成孩子上學(xué)時(shí)間長(zhǎng)短差異的原因在于，義務(wù)教育法對(duì)不同季節(jié)出生的孩子的影響。冬天的孩子早于其他孩子年滿16歲，這就意味著他們可以更早地合法離開學(xué)校，一些孩子正是這樣做的，因此導(dǎo)致這一群體受教育水平較低。

    這可能對(duì)所有此類研究都有效。但如果巴克爾斯和亨格曼所發(fā)現(xiàn)的模式是真實(shí)的，以往研究所提出的其他因素的重要性就值得懷疑。如果冬天出生的孩子更可能來自弱勢(shì)家庭，那他們命運(yùn)不如別人也是正常的了。

    上述兩位經(jīng)濟(jì)學(xué)家調(diào)查了美國(guó)疾病控制與預(yù)防中心（Centers for Disease Control and Prevention）提供的1989-2001年之間出生的5,200萬人的出生證明數(shù)據(jù)，這基本上是美國(guó)在此期間的所有出生人口。同樣的模式不斷出現(xiàn)：未婚媽媽、未成年媽媽和教育程度在高中以下的媽媽生育的孩子比例在每年1月達(dá)到高峰。舉例來說，在這13年中，1月份出生的孩子有13.2%母親只有十幾歲，而5月份出生的孩子這一比例為12%,二者相差不大，但在統(tǒng)計(jì)學(xué)上具有重要意義。

    亨格曼說，老實(shí)說，當(dāng)我們第一次發(fā)覺這種模式時(shí)，我們震驚不已，懷疑是不是弄錯(cuò)了。我們甚至一點(diǎn)都不興奮，而是覺得，這樣對(duì)嗎？

    他和巴克爾斯估計(jì)，對(duì)于個(gè)人間教育程度和收入差異，家庭背景因素所占的比例最多為50%.他們認(rèn)為，這表明安格瑞斯特和克魯格所描述的義務(wù)教育法的影響可能仍然存在，不過不能用以衡量學(xué)校教育對(duì)后來的收入有何影響，因?yàn)榧彝ケ尘昂蛯W(xué)校教育的影響仍然混雜，而沒有分開。