Families with twins.

Across the full Sassano-Bompietro tree — 23,153 individuals, 6,027 families — 200 sets of twins, and one set of triplets, are documented between 1671 and 1971, identified by siblings sharing an identical birth date. That's not a small number for a town the size of Sassano: at roughly one twin-birth family for every thirty families in the tree overall, it sits a little above the general historical rate of twinning, though not so far above it that the pattern speaks for itself. It needed a closer look.

The more interesting question isn't how many twin births there were, but whether twinning tends to run in particular families across generations, rather than appearing once and fading out. To get at that, every twin's own descendants were traced forward, generation by generation, checking whether any of them turned out to be a twin too. That search turned up 23 family lines where twins were born again later — sometimes to the original twin's own children, more often several generations further down the line.

The closest, cleanest case in the whole tree: Giovanni Libretti & Maria Spina's twins, Pasquale and Angela Rosa, born 19 Aug 1833 — and Pasquale himself grew up to father twins of his own, Rosa and Domenico Libretti, born 5 Feb 1881. Just one generation apart, and about as direct a hereditary signal as this kind of record can show. Most of the other 22 recurring lines stretch further — three, four, even six generations between one twin birth and the next in the same family line, which is exactly what you'd expect once a family has spread into enough branches that a repeat becomes statistically likely again on its own, hereditary tendency or not.

By sheer volume, the Libretti surname carries more twin-birth families than any other — 9 sets across the tree — followed by Feminella, Villano, Carlo, and d'Alessio at 7 each. Libretti also produced the closest one-generation recurrence, which makes it the single strongest candidate line if there is a real hereditary thread here worth following further.