When the Chinese government dictated that families limit themselves to one child each, it was a huge change: Chinese women averaged six births a piece in 1970, and parents traditionally relied on a large number of offspring to provide an economic security blanket. The purpose of the initiative, says Harvard Graduate School of Education Assistant Professor Vanessa Fong, was to help the country leapfrog from a Third-World economy into the First-World economy by mimicking First-World fertility and education patterns.   Now, 25 years later, China has its first generation of what Fong calls “singletons,” or only-children. These teens and young adults have had both education and the attentions of their parents lavished upon them, and, in urban areas, they have nearly universally been primed for good, white-collar jobs. The challenge, however, is that the economic opportunities in the country have not kept pace. As a result, the weight of expectation — both the expectations of the 20-somethings and the expectations of their parents — is, Fong says, nearly crushing. An anthropologist born in Taiwan and raised in California, Fong began her research in 1997, working with 107 families in the coastal city of Dalian as a tutor. In 1999, she started collecting data and narratives from 2,273 teens. The results of her study will be published in a forthcoming book, Only Hope: Coming of Age Under China’s One-Child Policy.