IN THE wake of the attacks on the Westgate Mall in Nairobi, speculation has grown about the possible involvement of Samantha Lewthwaite, a British convert to Islam. Ms Lewthwaite, known as the “White Widow”, was married to Germaine Lindsay, one of the London 7/7 bombers and himself a convert. She is wanted by Interpol and the Kenyan police in connection with a separate alleged bomb plot. Ms Lewthwaite, the daughter of a British soldier who served in Northern Ireland, grew up in the English home counties and converted to Islam in her teens. How common are converts to Islam? And why do they adopt the religion?

Those who embrace Islam tend to do so after years of contact with Muslims. (Ms Lewthwaite reportedly had a close relationship with Muslim neighbours during her youth.) Some, mostly women who make up around two-thirds of new believers in Britain, convert because they want to marry a Muslim. Others are fed up with what they see as the bawdiness of British society. Many speak of seeking a sense of community. Prisons have proven fertile ground for conversions for men. Some worry that those who convert in jail are exposed to more radical strains of Islam; others say that Islam's discipline and structure, along with the support they received from other Muslims, helped them to cope with life inside.

Calculating convert numbers is tricky. The census in England and Wales does not ask people about their past religions. British mosques do not keep a central record of conversions. Some new believers keep their conversions secret, worried about the reactions of friends and family. But using census data on race and religion, and questionnaires issued to mosques, Kevin Brice, a researcher at the University of Wales Trinity Saint David, calculated that around 5,200 Britons turn to Islam every year, and that the total number of converts is about 100,000. In America calculating conversion rates is even harder. The census does not ask about religion and few mosques keep registers of their members, so even the total number of American Muslims is uncertain, let alone that of converts. In 2007 the Pew Research Centre estimated that there were around 2.4m American Muslims; in 2000 President Bill Clinton made reference to a figure of 6m. Pew reckons that just under a quarter are converts, the majority of them African American.

Some worry that converts are more vulnerable than others to radical kinds of Islam because they know little about the religion's different traditions. That is not quite right, says Leon Moosavi, an expert on Islamic conversions at Liverpool University. The problem for converts is a lack of support, he argues. Some are abandoned by their families. They may not be accepted into mainstream mosques, many of which in Britain resemble ethnic clubs, he says. That isolation can make them vulnerable to extremists who hope that white converts will add credibility to their cause. But converts who turn to terrorism, as Ms Lewthwaite is suspected of doing, are rare. Indeed, the vast, peaceable majority may help to bridge the gap between Muslims and others. In Western countries the growth in converts is part of Islam’s transition from an immigrant religion to a home-grown one.