Previous research has suggested that gay and bi-curious men who meet online are more likely to indulge in unprotected sex and to have more partners than men meeting potential partners in other ways.
But since 2009 smartphone apps, such as Grindr, Scruff, and Recon, have become an increasingly popular way to hook up with potential sexual partners. They allow registered users to use their smartphone's GPS (global positioning system) to locate and network with other users in the vicinity.
Grindr, which was one of the first such apps, accumulated 2.5 million new users in 2012, and in 2013 reported that it had six million users in 192 countries worldwide.
The researchers wanted to find out if such use has altered behaviours and infection risk, so they collected data on HIV negative gay and bi-curious men attending a dedicated sexual health centre in Los Angeles, California, between 2011 and 2013.
In all, 7184 men were tested for sexually transmitted infections, and they provided information about their drug use and social networking methods to find potential sexual partners.
A third (34%) met sexual partners in person only; a slightly smaller proportion (30%) used a combination of person to person or online dating; and a slightly larger proportion (36%) used only smartphone apps or these plus other methods.
Smartphone apps tended to be favoured by younger (under 40) well educated men, and those of white or Asian ethnic backgrounds. App users were also more likely to use recreational drugs, including cocaine and ecstasy.
The analysis showed that men who had used smartphone apps to hook up with other men for sex were more likely to have common sexually transmitted infections than those who met their partners online or in clubs and bars.
They were 23% more likely to be infected with gonorrhoea and 35% more likely to be infected with chlamydia, although method of approach made no difference to the likelihood of infection with either HIV or syphilis.
The researchers suggest that smartphone apps make it easier to meet potential partners more quickly than online or more traditional methods, thereby boosting the chances of anonymous riskier encounters, and therefore of picking up a sexually transmitted infection.
They point out that their findings may not be applicable to gay men in other areas or to those not attending a dedicated sexual health clinic.
But they write: "Technological advances which improve the efficiency of meeting anonymous sexual partners may have the unintended effect of creating networks of individuals where users may be more likely to have sexually transmissible infections than other, relatively less efficient social networking methods."
And they add: "Technology is redefining sex on demand," they say. "Prevention programs must learn how to effectively exploit the same technology, and keep pace with changing contemporary risk factors for [sexually transmitted infections] and HIV transmission."