I don't buy much into supersitions and Friday the 13th and bad luck, so I'm glad there's research to back me up.
A more plausible hypothesis might be that Friday the 13th itself doesn't cause bad luck, but it does lead people to behave differently out of fear or anxiety, thereby causing things like accidents.
Subsequent studies, though, don't support this idea idea either.
While a Finnish study found that female drivers were more likely to get in accidents on the Friday the 13th, a subsequent follow-up found the correlation disappeared with better data and more controls. A German study similarly found that the number of accidents and errors that occur during hospital operations is pretty much the same on Friday the 13th as on other day