Yes, second cousins are blood related. You and a second cousin share a set of great-grandparents, which means you descend from the same couple just three generations back. On average, second cousins share about 3.4% of their DNA, though the actual amount can range from roughly 1% to 6% depending on how genetic material was shuffled during each generation.
How Second Cousins Are Connected
Two people are second cousins when their closest shared ancestors are a pair of great-grandparents. Here’s how that works in practice: your parent has a first cousin. That first cousin’s child is your second cousin. You’re both two generations below the first cousins, and three generations below the shared great-grandparents.
A quick shortcut for figuring out cousin relationships: count the number of “greats” in the title of your common ancestor and add one. Great-grandparent has one “great,” so 1 + 1 = second cousin. This only works when both people are the same number of generations away from that ancestor.
How Much DNA Second Cousins Share
According to data from 23andMe, second cousins share an average of 3.4% of their DNA. The range is wide, from about 1.1% to 6.3%, because of the randomness built into how DNA is passed from parent to child. Each generation, chromosomes are reshuffled, so some segments from a shared ancestor survive intact while others get broken up or lost entirely.
Despite that randomness, the chance of two biological second cousins sharing zero detectable DNA is vanishingly small, roughly 5 in a million. So if you and someone else are genuinely second cousins through biological descent, a DNA test will almost certainly pick up the connection. This makes second-cousin relationships reliably detectable through consumer genetic testing, unlike more distant relationships (fourth or fifth cousins) where sharing no DNA becomes common.
Second Cousins vs. First Cousins Once Removed
These two terms get confused constantly, but they describe different relationships. Second cousins are on the same generational level: you’re both the same number of steps from your shared great-grandparents. A first cousin once removed, on the other hand, is from a different generation. That person is either your parent’s first cousin or your first cousin’s child.
The “removed” part refers to generational difference. If your great-great-grandparent is someone else’s great-grandparent, you’re four generations from the common ancestor and they’re three. That one-generation gap makes you cousins “once removed.” The type of cousin (first, second, third) is determined by whichever person is closer to the shared ancestor.
In terms of shared DNA, a first cousin once removed typically shares more genetic material than a second cousin does, because one side of the relationship is only two generations from the common ancestor instead of three.
How This Compares to Other Relatives
To put second-cousin relatedness in context, here’s how the shared DNA drops with each step outward from your family tree:
- Siblings: ~50% shared DNA
- First cousins: ~12.5% shared DNA (share grandparents)
- Second cousins: ~3.4% shared DNA (share great-grandparents)
- Third cousins: ~0.78% shared DNA (share great-great-grandparents)
Each additional generation between you and your common ancestor roughly cuts the expected shared DNA in half. By the time you reach third or fourth cousins, the connection is thin enough that DNA tests sometimes can’t detect it at all.
Legal and Cultural Perspective
Because second cousins share a relatively small amount of DNA, marriage between second cousins is legal in virtually every jurisdiction worldwide. Legal restrictions on cousin marriage focus almost exclusively on first cousins, who share significantly more genetic material.
Consanguinity research defines “consanguineous marriage” as unions between second cousins or closer, which means second cousins sit at the outer boundary of what scientists consider a close biological relationship. In practice, the genetic overlap at 3.4% is low enough that it carries minimal additional health risk for offspring compared to the general population.
Even the recent wave of cousin-marriage legislation in Europe targets first cousins specifically. Norway banned first-cousin marriages in 2023. Sweden is expected to follow with a similar ban taking effect in 2026. Several U.S. states and East Asian countries also restrict first-cousin unions. None of these laws extend to second cousins.
What This Means for DNA Testing
If you’ve taken a consumer DNA test and matched with someone at around 1% to 6% shared DNA, a second-cousin relationship is one of the most likely explanations. Testing companies typically label matches in this range as “second cousin” or “second to third cousin,” though the exact label varies by platform.
Keep in mind that shared DNA alone can’t always distinguish between a second cousin and other relationships that fall in the same percentage range, like a first cousin twice removed or a half first cousin once removed. Building out your family tree to identify shared great-grandparents is the most reliable way to confirm you’re looking at a true second-cousin match.