The House of Habsburg occupied the throne of the Holy Roman Empire from 1438-1740.
Generations of intermarriage secured the family's influence across an European empire including Spain and Austria for more than 200 years but led to its demise when the final Habsburg monarch was unable to produce an heir.
"The Habsburg dynasty was one of the most influential in Europe, but became renowned for inbreeding, which was its eventual downfall," said lead researcher Roman Vilas, Professor at the University of Santiago de Compostela in Spain.
However, until now no studies have confirmed whether the distinct chin known as the "Habsburg jaw" was a result of inbreeding.
"We show for the first time that there is a clear positive relationship between inbreeding and appearance of the Habsburg jaw," Vilas said.
The new study, published in the journal Annals of Human Biology, combined diagnosis of facial deformities using historical portraits with genetic analysis of the degree of relatedness to determine whether there was a direct link.
The researchers also investigated the genetic basis of the relationship.
The researchers recruited 10 maxillofacial surgeons to diagnose facial deformity in 66 portraits of 15 members of the Habsburg dynasty.
Despite differences in artistic style, the portraits are characterised by a realistic approach to the human face.
The surgeons were asked to diagnose 11 features of mandibular prognathism, otherwise known as the "Habsburg jaw", as well as seven features of maxillary deficiency, the most recognisable of which are a prominent lower lip and an overhanging nasal tip.
The surgeons gave scores for the degree of mandibular prognathism and maxillary deficiency in each member of the Habsburg family.
Mary of Burgundy, who married into the family in 1477, showed the least degree of both traits.
Mandibular prognathism was most pronounced in Philip IV, King of Spain and Portugal from 1621 to 1640, said the study.
The extent of inbreeding was calculated from a large-scale family tree, including more than 6,000 individuals belonging to more than 20 generations.
Analysis was carried out to determine if it was connected to the degree of facial deformity.
The researchers detected a strong relationship between the degree of inbreeding and the degree of mandibular prognathism.
( With inputs from IANS )