Many parents are being put off having more than one child because of the " exaggerated warnings" about the costs, a think tank has said.
Mothers and fathers are being persuaded just to have one child because of fears over how expensive another would be , according to a book released by Civitas.
But research shows that children with siblings are more likely to be healthier and happier, argues journalist Colin Brazier in his book Sticking Up For Siblings.
The book highlights that the number of single-child families has risen from 18% in 1972 to 26% in 2007.
Parents should be wary of " PR-driven scare stories about the expense", the book argues.
A number of reports about the costs of parenting more than one child fail to take into account savings such as handed-down clothes and toys, shared lighting and heating, and a reduced spend on playdates, it says.
"Having a second child will not save a couple money, but it need not be as ruinous as the financial services sector suggests ," Mr Brazier writes.
"Annually, there is now a merry-go-round of media releases from banks, building societies and insurers, aimed at putting a figure on the cost of children. But these figures frequently tell only half a story. They fail to take account of the economies of scale produced by siblings; the handed-down buggies, clothes and cots, the shared holidays, heating and bathwater.
"Clearly, many parents do not rely on media scare-stories to form opinions on the economic viability of family expansion. Yet, it is strange to report that one of the most important and immutable decisions individuals make is informed by a niche area of social science that has been more or less colonised by PR execs from the City."