A new study published in the American Journal of Human Biology has uncovered a significant relationship between family intermarriage and later-life outcomes in one of Europe’s most influential royal houses. Researchers focused on the House of Habsburg, a family famous for multiple generations of marrying close relatives. They found that individuals with higher levels of inbreeding were more likely to die at a younger age, and that inbreeding was linked to an unexpectedly high risk of mothers dying shortly after giving birth. These results suggest that marrying close relatives can have an impact beyond childhood, affecting both how long someone lives and, in some cases, whether a mother survives after having a baby.
The investigators decided to look at the House of Habsburg because it provided a rare opportunity to see the effects of inbreeding in a consistent social environment. Members of a royal family typically enjoyed the same high standard of living, advanced medical attention for their time, and abundant resources. This uniformity helps reduce the chance that differences in wealth, diet, or health care would muddle the results, making it easier to detect any genetic factors related to marrying relatives.
The Habsburg family tree also features a wide range of inbreeding levels. Some individuals had very little shared family background, while others were as closely related as an uncle and niece. This range allowed the researchers to compare people across a broad spectrum of inbreeding, giving them a fuller view of how inbred ancestry might influence health and survival.
To carry out the study, the authors reconstructed a detailed genealogy of the Habsburg dynasty from approximately 1450 to 1800. They gathered birth, death, and marriage data from multiple historical sources, entering more than 8,000 individuals spanning over 20 generations into a database. This approach enabled them to calculate a specific “inbreeding coefficient” for each person and a “kinship” value for each marriage. These calculations quantify the chance that a child inherits two copies of the same gene from a common ancestor. Some marriages were as close as first-cousin unions, while others fell somewhere between second cousins and unrelated pairs.
The researchers then looked at how these numbers related to longevity, maternal mortality (death shortly after giving birth), and fertility outcomes such as the number of children and the spacing of births. By applying statistical models that included time-to-event (survival) analyses and special methods for count data, they made sure they could account for zero-child outcomes and deaths at different ages.
Their findings shine new light on the broader impact of inbreeding. In particular, the study showed that women with higher inbreeding coefficients tended to die younger, but this was only obvious among those who had at least one child. This link hinted that problems during pregnancy or labor might be more frequent or more severe for women with close-relative ancestry.
When the authors investigated how many women died during the four-week period after giving birth, they made a striking discovery: the more inbred a mother was, the higher her chance of dying in that first month postpartum. About one out of five mothers in the sample did not survive that early period if they fell into the more inbred categories, a proportion far above typical modern rates.
Historically, giving birth was far riskier than it is today, but these results suggest that women with closer-kin ancestry may have faced even greater hazards. Although the exact medical causes of death remain unknown, it is plausible that heightened vulnerability to infections or complications during labor could have contributed to these outcomes. Previous research has shown that inbreeding may raise the odds of having harmful recessive genes, which can increase susceptibility to serious illnesses.
The study also looked at fertility, focusing on how many children a woman had and how far apart those births occurred. The most inbred women did not necessarily have fewer total children, in part because royal women faced significant social pressure to ensure at least one surviving heir. They may have tried repeatedly to become pregnant if a child was lost in infancy. This pattern could be a form of “reproductive compensation,” where more inbred women kept attempting to have a child in order to secure the future of the dynasty.
However, the researchers did detect a link between higher inbreeding levels in women and longer intervals between births. One possible explanation is that such mothers were less physically fit after childbirth, making it more challenging for them to conceive again right away. In that sense, inbreeding might have lowered overall female fertility potential, but strong social expectations to produce heirs could mask that effect by encouraging repeated attempts at pregnancy.
Every study of this kind has its limitations, and the authors acknowledge several important considerations. Many of the family records they relied on were hundreds of years old, and details about the exact causes of death might be incomplete or inaccurate. Historical records also do not fully capture the everyday conditions individuals faced, including the quality of any medical interventions or the possible presence of hidden illnesses. The broader environment also changed over the three and a half centuries covered in the analysis, so certain differences in survival could reflect shifts in nutrition or medical practices.
Future researchers may want to compare these findings with other royal families that practiced cousin marriage or investigate modern populations where inbreeding remains common. It would also be interesting to see whether contemporary medical care lowers the impact of close-relative ancestry on maternal risk.
The study, “Inbreeding Effect on Maternal Mortality and Fertility in the Habsburg Dynasty,” was authored by Francisco C. Ceballos, Román Vilas, and Gonzalo Álvarez.
