Archaeologists have long made a comfortable assumption about graves holding an adult and a child together: that they must be family. A new study from Stockholm University suggests that, in medieval Sweden at least, they usually weren't.

Researchers analysed ancient DNA from 142 people who lived from the late Viking Age into the early Christian Middle Ages. The remains came from three Swedish sites: the urban cemetery at Sigtuna, the rural churchyard of Västerhus in Jämtland, and Fjälkinge in Skåne. Sixty-eight of them were children or adolescents, many buried in shared graves with adults.

"We often assume that adults and children sharing a grave were parents and children or other close family members. In most cases, that was not what we found," says Maja Krzewińska of the Centre for Palaeogenetics at Stockholm University, lead author of the study, published in the journal Science Advances.

Close biological relatives, the genetic evidence showed, were surprisingly rare among people placed in the same grave. That held true even in cemeteries where DNA testing found plenty of family ties elsewhere in the wider burial ground. Something other than blood was often deciding who ended up sharing a grave.

How scientists read a 1,000-year-old family tree

Ancient DNA, or aDNA, works by extracting tiny fragments of genetic material that survive in old bones and teeth. Scientists then compare genetic markers between individuals to establish how closely related they were: parent and child, siblings, cousins, or no relation at all.

"Archaeologists have debated the relationships between people buried together in this type of grave for a long time. Ancient DNA has finally given us the tool we have been waiting for to test these interpretations directly," says Anna Kjellström, a researcher in Stockholm University's Department of Archaeology and Classical Studies.

The method proved especially useful for children. From the 10th and 11th centuries onward, Christian burial practice largely did away with grave goods. Bodies were laid out simply, leaving little to help archaeologists judge sex from bones alone, particularly in young skeletons. DNA filled that gap. The results showed boys and girls were buried according to the same customs as adult men and women. At Västerhus, where men and women were generally buried on opposite sides of the churchyard, children followed the same pattern by sex.

"The children were not treated as a separate category. In death, they appear to have been treated according to the same social and religious principles as adult men and women," says Anders Götherström, Professor of Molecular Archaeology at Stockholm University.

The pilgrim in grave 56

Among the Västerhus burials, one skeleton stood out. Lady 56 died before the age of 30 and was buried with a scallop shell. It is an object rarely found in Scandinavian graves: only two have ever turned up at Västerhus. Across medieval Europe, though, the shell was instantly recognisable as the emblem of a completed pilgrimage to Santiago de Compostela, more than 3,000 kilometres away in north-west Spain.

Unlike many of the study's grave-mates, Lady 56 really was surrounded by family. DNA linked her to her parents, a brother and two daughters, all buried in different parts of the same churchyard. Hers is one of the cemetery's most richly documented households. One member of it, it seems, made an extraordinary journey across the continent and back before dying at home in Jämtland.

Rethinking medieval "family"

Taken together, the findings suggest that in these Swedish communities, the choice of who shared a grave often had less to do with bloodlines than with social, religious or community ties.

The researchers frame this as a finding about their sample, not a verdict on medieval Europe as a whole. But for these three Swedish cemeteries, it's a reminder that "family," in death as in life, could mean something broader than genetics.

As Stockholm University puts it, archaeogenetics can reveal far more than biological family relationships. Combined with archaeological evidence, it offers a new window onto how medieval communities organised themselves, raised their children, and laid their dead to rest.