Trolls, companies which develop technology but don't use it to make products, claim that they are advancing technological progress.
Top troll Nathan Myhrvold, who left Microsoft to form a troll company called Intellectual Ventures, argues that it's hard for inventors to get paid for patents and that trolls help them to get paid.
BostonUniversity disagrees, saying that trolls actually take more out of the US economy than they put in.
2,150 US companies had to defend 5,842 patent lawsuits last year, says the university.
Moreover, the $29 billion figure does not take into account the cost of the diversion of companies' resources to deal with trolls, the consequent delay in product introductions and the resulting loss of market share.
The $29 billion does take into account the cost of lawyers' fees, licensing fees and royalties. Lawyers' fees represent a quarter of the cost of the litigation, says the university.