According to Wikipedia ‘clubbed thumb’ used to be a sign of descent from European royal blood lines. Could there be any truth to this? According to the science – as a way of judging blood lines the clubbed (or stub) thumbs trait might be ideal because of its expression (i.e. it shows up relatively often in the few families who have it).
I have found out that the technical name is brachydactyly type D (BDD). The clubbed thumb is defined as a shortening and broadening of the the end bone or phalanx of the thumb. They are also trying to determine if there is a gene for the trait.
It is a common trait, aproximately 1 million Americans will have clubbed thumbs (occurring in 0.4% of whites and 0.1% of blacks in the US). In Israel it occurs in 1.6% of Jews and 3% of Arabs. It is also found a lot in Japan (this info from the book Abnormal Skeletal Phenotypes, which you can find on google books (http://books.google.co.uk/books?id=oG5Fn4Ucus4C).