The global drop in fertility has a number of causes, but rising biological infertility (infecundity) is not one (Toxins plus climate harms likely cause of reduced fertility, study finds, 26 April). Recent reports of a paper by Shanna Swan, the writer of Spermageddon, and others have claimed the paper shows that exposure to pollutants has been driving down biological fertility and may be contributing to the downturn of fertility rates in recent years. This is unlikely.

A meta-analysis published last year, which controlled for regional variation, found that sperm counts increased in the US in recent years. Although there are other mechanisms by which biological fertility could be affected, time to pregnancy (TTP) directly tracks how quickly couples conceive. TTP increased in Britain in the late 20th century, and has been stable between 2002 and 2017 in the US for women under 30, only increasing by about 4% for women who already had a child. Meanwhile, infertility has been staying around the same or decreasing in developed countries in recent years.

This suggests that trends in pollution-related damage to fertility, if they exist at all, are not responsible for recent falls in fertility rates. Explanations for recent trends would be better off looking at the turning point: across much of the western world, fertility rates peaked around 2007-10, and have been in decline since. We would therefore be better off examining economic conditions and smartphone usage (and its effects on coupling) as likely causes of the fertility bust.
Peter Foreshaw Brookes
Director, The Centre for Family and Education