THE ever-rising proportion of births outside wedlock is one of the great social trends in postwar England and Wales. Since 1958 it has shot up from 5% to 47%. But from 2007 to 2013 the growth slowed; last year the proportion even dipped fractionally. Why? One explanation is the ageing of motherhood, suggests Ann Berrington of Southampton University. Teenage childbearing, which usually happens out of wedlock, fell from 26.6 live births per 1,000 women in 2006 to 17.4 in 2013.

Another reason is immigration, she says. Immigrants tend to be of child-bearing age and, on average, have more children than women born in Britain. The percentage of births to foreign-born women increased from 16.5% in 2001 to 26.5% in 2013. Many migrants have traditional views on marriage and childbearing. This helps explain why London, where 36% of residents were born outside Britain, has the lowest percentage of unmarried mothers in England and Wales (see map).

Within London, the boroughs with the highest percentage of unmarried mothers are the comparatively poor, white ones in the east and on the outskirts. Boroughs with many immigrants tend to have much lower proportions of unmarried mothers, as do the wealthy boroughs of west London.

As immigrants move to the suburbs, so families become more traditional there. Between 2011 and 2013 the share of unmarried births in Barking and Dagenham, Merton, Bromley and Havering—some of London’s whitest suburbs—fell. This coincided with a decline in the proportion of British-born white residents in those places.

Oddly, the percentage of unmarried births in Tower Hamlets went up in the same period. This may be because of the changing composition of the growing borough population. But the uptick may also be because a few more Bangladeshis (who make up 32% of the population) are having children out of wedlock. Across England and Wales the proportion of births to unmarried Bangladeshi-born women doubled to 4.5% between 2008 and 2012. Immigrants may gradually be taking on some native ways.