Second cousins, step by step
Every cousin relationship starts with one question: which ancestors do you share? For second cousins, it's your great-grandparents.
Walk up your family tree three steps: you, your parent, your grandparent, your great-grandparents. Now walk back down a different branch. Your great-grandparents had another child (your great-aunt or great-uncle). Their child is your parent's first cousin. And their child is your second cousin.
Put simply: if your mom and their dad are first cousins, you two are second cousins.
The mix-up everyone makes
Your first cousin's kid is not your second cousin. That's your first cousin once removed, because you're from different generations. "Second" is about how far back the shared ancestor is. "Removed" is about generations apart. Here's the full explanation of "removed".
| This person… | …is your |
|---|---|
| Your parent's first cousin's child | Second cousin |
| Your first cousin's child | First cousin once removed |
| Your parent's first cousin | First cousin once removed |
| Your second cousin's child | Second cousin once removed |
| Your parent's second cousin | Second cousin once removed |
| Your child and your first cousin's child | Second cousins to each other |
What is a second cousin once removed?
Either your second cousin's child, or your parent's second cousin. You're one generation apart, and you share about 122 cM of DNA on average. Twice removed means two generations apart, like your grandparent's second cousin.
How much DNA do second cousins share?
About 3%, or 229 centimorgans (cM) on average according to the Shared cM Project. The range is wide, from around 41 to 592 cM, because DNA is passed down in random chunks. Almost every pair of second cousins shares some detectable DNA, which is why consumer tests find them so reliably. Your match list will usually label them "2nd–3rd cousin." Check any shared cM number here.
How many second cousins do you have?
More than you think. If every couple in your family had two kids, you'd have about 4 first cousins and about 16 second cousins. Older generations had bigger families, so plenty of people have 50 or more second cousins and have never met most of them.
That's exactly how two people end up dating without realizing they share great-grandparents, especially when families moved, changed names or lost touch.
Can you marry your second cousin?
Yes, in every US state, and in almost every country. The extra genetic risk for children is small, under one percentage point above the roughly 3% baseline every couple has. See cousin marriage laws by state.
The bigger question for any couple, related or not, is whether you both carry the same hidden recessive condition. That's what a carrier check answers.