Ask a couple whether they might be related and you'll usually get a laugh, a quick glance at each other, and the same answer: "Pretty sure we're not."
It's a perfectly honest answer. It's also not an answer at all.
Your family tree has limits
Family trees are written by people, and people forget, simplify and occasionally keep secrets. Adoption records get sealed. Families move continents and change their names. A great-uncle's other family never makes it into the photo album. Grandma knew a lot. She didn't know everything.
For most of history, none of that mattered much. You married someone from the next village, and the next village knew exactly who your cousins were. That world is gone.
Anything is possible in this day and age
Today you can meet your person on an app, in a city neither of your families has ever lived in. Your cousins could be anywhere. So could your half-siblings — including ones you've never heard of.
Some sperm donors have 50 to 200 known children, and the US sets no legal limit on how many families can use one donor.
Adopted and donor-conceived people often don't know who their biological relatives are — and neither do the people dating them.
One Colorado couple took DNA tests out of curiosity and learned they were cousins — after 17 years of marriage and three kids. Neither family had a clue.
None of these people did anything wrong. They just had the same answer everyone has: pretty sure we're not.
"Pretty sure" is a fine answer at a dinner party. It's a weak one at a wedding.
Grandma's system was good. It just had blind spots.
Plenty of cultures solved this long before DNA. Hindu families check gotra, and classical texts rule out marriage within seven generations on the father's side and five on the mother's. Orthodox Jewish communities have run confidential premarital matching through Dor Yeshorim since 1983. Iceland, where everyone is somebody's cousin, built an app on top of its national genealogy database.
These systems are smart. But they run on records and memory, and gotra only follows the father's line. Your mother's sister's son has a different gotra — and he's still your first cousin.
DNA doesn't care whose side of the family it came from. It reads all of them.
What a DNA check actually tells you
When two people compare DNA, the result is a single number: how many centimorgans (cM) of DNA you share. The bigger the number, the closer the relationship.
That's it. It isn't a medical test, and it doesn't predict your future kids' health — that's carrier screening, a different test. A relatedness check answers one question, properly: are we related, and how closely?
So — are you related?
Maybe you're 100% sure. Great. If you've both already done a consumer DNA test, a check takes about a minute, and you get to say "we checked" instead of "pretty sure."
And if you're not sure? That's exactly what it's for.
Already have your shared cM number? Try the free calculator. Haven't tested yet? Grab a spot in the beta.