Torfason used the Corruption Perceptions Index and compared it with a separate index on tipping, which looks at 33 professions (such occupations as hairdressers and cabdrivers) and whether its customary to tip those workers in a given country.
He found that as the prevalence of tipping behavior went up, so did a country's place on the Corruption Perceptions Index. Here's what that looks like in graph form. The number of professions traditionally tipped is on the X axis, ranking on the corruption scale is to the left.
We suggest that tips and bribes both emanate from similar norms of exchange—indeed, the timing of the gratuity may be the key distinguishing feature between these two acts. This subtle temporal distinction may help explain why tipping and bribery practices are positively correlated across countries even though many individuals perceive them as diametrically opposed from a moral standpoint.
Read the full paper (ungated) here.