The best time to rob a bank is "in the morning", according to one of the authors of Freakonomics.

But bank robbing's return on investment is "among the worst in crime", said Stephen Dubner.

On the publication of their multi-million selling pop-economics book 10 years ago, Dubner and Freakonomics co-author Steven Levitt set up a blog to explore the stranger aspects of our economic behaviour.

Now, 8,000 blog posts on, they've released a book of the best - When to Rob a Bank - which ponders, among other topics, the rate of return of bank robbery and the possibility of a sex tax.