Couples are restricting the size of their families in the.UK because of cash worries brought on by the financial crisis and the subsequent decline. We're now up to nearly 3. 7 million families where there is an only child, a rise from about 3. 3 million in 2005. That means nearly half of all parents have only one child.Financial worries aren't the only driver. The trend towards later motherhood has been mentioned as a cause, as have soaring costs of raising a child, which have been calculatedas￡222.500 from birth to 21 years of age. This is an increase of nearly 40% in 10 years.The increasing availability of IVF（试管婴儿）is also a factor and an interesting one. Couples who might have remained childless in the past now invest in IVF and get pregnant. And because of the cost they stop after one child.It may not be a bad thing; there are outstanding examples of talented only children. Some argue that being an only child promoted their success. These include actors Natalie Portman and AI Pacino, golfer Tiger Woods and even Queen Victoria. A study from the Institute for Social and Economic Research at the University of Essex also showed that the fewer brothers and sisters a child has, the happier they are. It seems fighting for parental attention and affection—which sometimes descends into physical fights—is more stressful than any adult had previously thought. And it's not compensated（弥补）by having a playmate.What is a proved advantage of one-child family?