单选题 Couples are restricting the size of their families in the UK because of cash worries brought on bvthe financial crisis and the subsequent decline.We’re now up to nearly 3.7 million families whelthere is an only child，a rise from about 3.3 million in 2005.That means nearlv half of all parentshave only one child. 

Financial worries aren’t the only driver.The trend towards later motherhood has been mentionedas a cause，as have soaring costs of raising a child，which have been calculated as￡222.500 frombirth 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.Coupleswho might have remained childless in the past now invest in IVF and get pregnant.And because ofthe cost they stop after one child. 

It may not be a bad thin9;there are outstanding examples of talented only children.Some arguethat being an only child promoted their success.These include actors Natalie Portman and Al Pacino，golfer Tiger Woods and even Queen Victoria.A study from the Institute for Social and EconomicResearch at the University of Essex also showed that the fewer brothers and sisters a child has.thehappier they are.It seems fighting for parental attention and affection--which sometimes descendsinto physical fights-is more stressful than any adult had previously thought.And it's not compensa-ted（弥补）by having a playmate. 

The smaller size of UK families is mainlv related to __________ .