The enduring mystery of the fertility decline has a new culprit: the smartphone.

生育率下降这个长期未解之谜如今又出现了一个新的罪魁祸首：智能手机。

Experts have long wondered if phones played a role in the birthrate decline — which began in 2007, the same year that Apple introduced the iPhone — but until now there had not been hard evidence to prove it.

长期以来，专家们一直怀疑手机可能在出生率下降中发挥了作用——因为出生率下降恰好始于2007年，这一年正是苹果推出iPhone的年份，但此前一直缺乏能够证明这一点的确凿证据。

Two new papers, one published Monday and the other in May, are the first academic endeavors that test whether the smartphone was a cause.

今年5月和本周一相继发表的两篇新论文是首批尝试检验智能手机是否为原因之一的学术研究。

They are the most recent efforts to explain the sweeping fertility rate decline in the United States and other countries over the past 20 years. Researchers have already looked at contraception use, abortion rates, rising levels of female education and even the popular television show “16 and Pregnant.”

这是研究人员近年来为解释过去20年美国及其他国家生育率全面下降所做的最新尝试。此前，他们已经考察过避孕措施使用情况、堕胎率、女性受教育程度提高，甚至连热门真人秀《少女妈妈》都被纳入研究范围。

Proving phones caused the decline is a tricky endeavor. There were a number of major events in those years, including the Great Recession, and isolating smartphone use is difficult.

要证明手机导致生育率下降并非易事。包括全球金融危机在内，那些年发生了许多重大事件，将智能手机的使用单独剥离出来加以分析难度相当之大。

The gold standard for scientific evidence is known as random assignment. It compares outcomes for people randomly chosen to receive a treatment (like getting a smartphone) with people who are not.

科学证据的黄金标准是随机试验，也就是将随机选出的接受某种干预（如获得智能手机）的人群与未接受干预的人群的结果进行比较。

But that is not possible when it comes to ferreting out reasons for declining fertility.

但在追寻生育率下滑的原因时，这种方法并不适用。

So researchers sought out data about smartphones that introduced randomness.

于是，研究人员转而寻找与智能手机相关、能够引入随机性的数据。

Caitlin Myers, an economist at Middlebury College, and Ezekiel Hooper, her student, used the spotty early rollout of the iPhone as a way to isolate the effects of the phone on fertility. The first iPhone was released in June 2007, they wrote, and was available only on the AT&T network until February 2011. The study compared fertility rates in U.S. counties that had near-universal AT&T coverage with counties that had little or none.

明德学院的经济学家凯特琳·迈尔斯和她的学生以西结·胡珀利用iPhone早期推广覆盖面的参差不齐剥离出手机对生育率的影响。他们在论文中写道，第一代iPhone于2007年6月发布，直至2011年2月前只能使用AT&T网络。研究比较了AT&T信号几乎全面覆盖的美国县与几乎没有覆盖地区的生育率变化。

Their paper, published in the National Bureau of Economic Research, found that the iPhone caused as much as half of the fertility decline between 2007 and 2011. The most pronounced effects were among young people aged 15 to 24.

他们发表于国家经济研究局(NBER)的论文发现，iPhone造成了2007年至2011年间生育率下降的一半左右。其中影响最明显的是15至24岁的年轻群体。

What happened in the counties with iPhones? One theory, Professor Myers said, is that young people began to socialize more on their phones and less in person, and consequently were less likely to have sex and become pregnant.

那么，在iPhone普及的地区究竟发生了什么？迈尔斯教授说，一种理论认为，年轻人开始更多地通过手机进行社交，减少了面对面交往，因此发生性行为和怀孕的可能性也随之降低。

Professor Myers said iPhones may also have made pornography more accessible, which led young people to substitute it for sex, or young people may have used them to obtain better information on avoiding pregnancy, including contraception and abortion.

迈尔斯教授表示，iPhone可能也让色情内容更容易获取，导致年轻人用它替代了性行为，也可能因为年轻人能够通过手机获得了更多避免怀孕的信息，包括避孕措施和堕胎。

Researchers not involved in the study said the results were persuasive.

一些未参与该研究的学者认为，这些结果颇具说服力。

Phillip B. Levine, an economist at Wellesley College, said he was “a little jealous” of the Middlebury data, which he said provided a real insight into a potential driver of a major social change.

韦尔斯利学院的经济学家菲利普·莱文表示，他对明德学院使用的数据“有点嫉妒”，认为这些数据为理解一项重大社会变迁的潜在驱动因素提供了真实洞见。

He said some variation in the AT&T data could throw off the final finding. For example, the company may have set up in counties that were wealthier, or more densely populated, introducing a pattern “that’s not likely to be random anymore,” he said.

不过他也指出，AT&T覆盖数据中存在某些差异，可能会影响最终结论。例如，该公司可能优先进入那些更富裕或人口更密集的地区，从而形成一种“不再具有随机性”的模式。

He said that Professor Myers tried to account for those variations, and that her findings made sense.

他表示，迈尔斯教授已尝试对上述差异加以修正，而且研究结果是合理的。

But, he cautioned: “You shouldn’t take the result so literally and say: Oh, it’s the iPhone’s fault. It’s an example of the kinds of social influences that have led to the decline in birth rate.”

但他同时提醒说：“你不应该过于从字面理解这个结果，然后说：‘哦，都是iPhone的错。’这只是导致出生率下降的诸多社会影响的一个例子。”

Dropping birthrates, once a feature of rich societies, are now a near-global phenomenon. The sweep of the decline has researchers looking for common drivers. The authors of the second study also decided to look at smartphones.

出生率下降曾经是富裕社会的特征，现在已近乎成为全球现象。这种下降的广泛性促使研究人员寻找共同的驱动因素。第二项研究的作者也将目光投向了智能手机。

“Countries with very different health care systems, welfare regimes, abortion laws, religious traditions, recessions and demographic trends all saw similar breaks in the same window,” wrote the authors, Hernan Moscoso Boedo, an economics professor at the University of Cincinnati and Nathan Hudson, a Ph.D. student.

来自辛辛那提大学的经济学教授埃尔南·莫斯科索·博埃多和博士生内森·哈德森写道：“医疗体系、福利制度、堕胎法律、宗教传统、经济衰退情况以及人口结构都截然不同的国家却在同一时间段经历了类似的生育率断崖式变化。”

“Whatever caused it was something global — something that arrived in roughly the same form in all of these places at roughly the same time,” they wrote.

“因此，导致这一现象的原因很可能是一种全球性的因素——某种几乎同时以相同形式出现在世界各地的东西。”

They analyzed World Bank data measuring smartphone prevalence and teenage fertility rates in 128 countries. In countries as varied as Iran, Costa Rica, Guatemala, Chile, Mexico and Turkey, they found that teenage fertility declines accelerated once smartphones became a mass phenomenon.

他们分析了世界银行关于128个国家的智能手机普及率和青少年生育率的数据。他们发现，在伊朗、哥斯达黎加、危地马拉、智利、墨西哥和土耳其等差异巨大的国家，一旦智能手机成为一种大众现象，青少年生育率的下降就会加速。

They tested their technology theory in the United States using data on wired broadband and 4G high-speed mobile networks. They looked at where access was better and worse and found a substantial effect: that fertility rates for teenagers declined fastest in counties with more high-speed access.

他们还利用美国有线宽带网络和4G高速移动网络的数据对这一技术假说加以验证。通过比较网络接入条件优劣不同的地区，结果发现了显著影响：高速网络接入更为普及的县域，青少年生育率下降最为迅速。

Theodore Joyce, an economist at Baruch College, said he was skeptical of both studies. Teenage births have been falling since the 1990s, he said, long before technology came on the scene. Professor Myers’s paper, he said, examined a short period before smartphones had fully taken hold.

巴鲁克学院经济学家西奥多·乔伊斯表示，他对两项研究均持怀疑态度。他指出，青少年生育率自1990年代起便已持续走低，远早于智能手机时代的到来。他还说，迈尔斯教授的论文所考察的时间段较短，当时智能手机尚未全面普及。

The hypothesis, he said, could be correct but “remains speculative.”

他说，这种假说或许是正确的，但“目前仍然属于推测”。