FRIDAY, May 8 (HealthDay News) -- Members of the powerful Habsburg royal family ruled Spain for centuries until the dynasty died out at the beginning of the 18th century.

Now, a new study suggests the reason for their decline lies not in the stars but in themselves -- and their unfortunate habit of marrying their own relatives.

Ultimately, the genetic havoc caused by inbreeding appears to have doomed the Spanish Habsburgs to oblivion via infertility, scientists report in a research paper released last month.

"Inbreeding was a major cause responsible for the extinction of this dynasty" and also contributed to the deaths of many Habsburg children, said study co-author Francisco C. Ceballos, a researcher at the Universidad de Santiago de Compostela in Spain.

The study suggests that successive inbreeding contributed to the genetic diseases that turned the last Habsburg king, Charles II, into a mental and physical cripple who was unable to have children. Charles II reigned from 1665 until his death in 1700 at the age of 38.

The findings appear in a recent issue of the journal PLoS ONE.

Today, the Habsburgs are most well known for their domination of Europe over three centuries -- Charles II ruled not only Spain but Italy and much of what is now the Netherlands -- and for their oddly protruding jaws.

In Spain, the Habsburgs ruled from 1516 until 1700, when Charles II died without offspring, despite two marriages.

The family faced a challenge because they needed to marry Catholic spouses of equal rank -- a rarity -- and because dynastic marriages were used to keep territories within the family's grasp, explained Alan Sked, a historian at the London School of Economics and Political Science.

In their study, Ceballos and colleagues examined the Habsburg family tree through more than 3,000 people, going back 16 generations. They aimed to figure out how inbreeding -- cousins marrying cousins, for example, and uncles marrying nieces -- may have affected the family's health.

The team found that the final Habsburg ruler, Charles II, had a high chance of developing genetic abnormalities due to inbreeding. Indeed, he was physically stunted and suffered from a variety of intestinal and blood ailments, so much so that his subjects dubbed him "El Hechizado," or "The Hexed."

The study also suggests that interbreeding weakened the family's health so much that it contributed to an extraordinarily high death rate among children. From 1527-1661, 10 of 34 children in the Spanish Habsburg family died before the age of 1, and another seven died before they were 10. The researchers note that the death rate was much higher than would even be expected for children born in that era.

Overall, the Habsburgs' habit of inbreeding led to family members sharing up to 20 percent of identical genomic material, the researchers said.

The conclusion that inbreeding may have hurt the Habsburg family's health is a reasonable one because the odds of genetic disease rise when people marry their relatives, according to Andrew J. Bohonak, an associate professor of biology at San Diego State University.

Possibly as a result, "there are social taboos against inbreeding in most human societies," he said. "One of the few places you see these taboos avoided is in some of the royal families of old."

What would have happened if the Habsburgs hadn't married each other? Sked, the historian, said "there would have been changes in alliances, boundaries and policies. Most of all, the Habsburgs would have produced more capable and intelligent rulers."

More information

Learn more about the Habsburgs from the Encarta encyclopedia .