The US is responsible for nearly half of the world's spam, according to security firm Symantec. 

In a study of junk email received during the second half of 2006, Symantec found that roughly 45 per cent of the messages were sent from computers within the US.

The volume of spam coming from the US was more than seven times that of China, the second largest purveyor.

"This is not to say that the spammers themselves are American," Symantec security researcher Nick Sullivan said on the company's security response blog. 

"The purveyors of illicit pharmaceuticals, gurus of pink sheet penny stocks, and so-called representatives of 'your bank' may very well be from China, Russia, Brazil and other countries, but the spam itself is sent mostly through American computers."

The most likely cause of the high rate of stateside spam, according to Sullivan, is that spammers prefer to use American ISPs and free email services (such as Hotmail or Yahoo Mail) to spread junk mail.

He also suggested that US botnets are specialising in sending spam because of the proliferation of broadband internet accounts that make it easier to send large volumes of spam messages.

Sullivan admitted, however, that this theory has flaws. Some regions in Western and Central Europe have much higher broadband penetrations than the US, yet account for a relatively small percentage of spam.