Banks and credit unions collected $23.7 billion in overdraft fees in 2008, up 35.4% from 2006, according to a study published this month by the Center for Responsible Lending.

Banks and credit unions charged an average of $34 per overdraft, often higher than the purchase that triggered it, the study found.

Leslie Parrish, a senior researcher at the center, said the higher fees underscore the widespread use of debit cards. According to data the center published in 2005 and 2006, debit cards trigger 44% of overdraft fees; automated clearing house payments are responsible for 28%; checks, 27%; and account-maintenance fees, 1%.

The most common triggers of overdraft fees in 2008 were small debit card purchases ranging from $5 to $20, the report said.

"Today, nearly three-quarters of checking account customers have a debit card, with active card users averaging 17 debit card transactions per month," the report said. Debit cards "increased the number of overdraft events, while the customer was unaware he had crossed the line."