The decline of modern day fertility

Jan 24 2008

Researchers at the University of Sheffield have shown that mothers are choosing to have fewer children in order to give their children the best start in life, but by doing so are going against millenia of human evolution.

The research sheds new light on the decline of modern day fertility.

Researchers Duncan Gillespie, Dr Virpi Lummaa and Dr Andrew Russell, all from the University's Department of Animal and Plant Sciences, studied Finnish church records from the 18th and 19th centuries and traced the reproductive histories of 437 women, their 2888 children and 6470 grandchildren.

They found that fertility of children from large poor families appeared to be constrained, potentially due to a lack of wealth and resources. Children from large wealthy families, on the other hand, went on to have large families themselves. According to the research this has caused a trade-off between offspring quantity and quality with modern women choosing to have fewer children so that they can instead invest in their education or career, gaining resources to give their children the best start in life.

The researchers also found evidence for an evolved relationship between a mother's fertility and the fertility of her children – the more offspring a woman has, the larger her overall family will be. This means that women having fewer children will ultimately have fewer grandchildren.

Duncan Gillespie said: “Before modern day birth control high fertility was a sign of wealth and families would therefore strive to have large numbers of children. However for poor mothers, having more children did not always lead to more grandchildren, due to economic constraints on their children's fertility.

“In today's society, this has gone even further with wealthy families choosing to invest in fewer children as well. However, this trade-off between offspring quantity and quality has come full circle in that fewer children will ultimately lead to smaller families. This could help explain the decline in fertility in modern society.”