So if you’re American you’re probably eating Thanksgiving this Thursday with your close friends and family. And you’re probably pretty certain you can recognize your siblings right? But what makes you so certain they are genetically connected to you?

"We take it for granted how we know who are mother is, how we know who our father is, how we know who our brothers and sisters, aunts and uncles, cousins, etc."

That's David Lewis a PhD student in psychology at the University of Texas at Austin. And he recently completed a study specific to sibling recognition.

Evolutionary psychologists point to these two things to tell if you and your sibling share the same mom: witnessing your sibling having a maternal perinatal association otherwise known as “breastfeeding.”

"So if you're an older sibling, watching a younger individual nurse from your mother is a very reliable cue to being siblings with that individual because in mammals, mothers have 100 percent maternity certainty. That is to say that aside from modern inventions with surrogate mothers, mothers are 100 percent certain that the thing that they give birth to is their biological offspring."

And the other variable is length of co-habitation with your sibling and mother.

But sibling recognition also requires a known shared paternal connection.

"But there's a problem in mammals. And that is that for fathers they cannot be 100 percent certain that their mates' offspring are their genetic progeny. Unless they did a DNA test. But unfortunately during the course of human evolution they didn't have those handy DNA tests."

So if fathers cannot be certain that their offspring are theirs then siblings cannot be certain they share the same father.

Lewis wanted to find out if facial resemblance could be a cue to genetic relatedness. Meaning that the more siblings resembled each other, the more certain they could be that they shared mom and pop genes, and so also more likely to have a close relationship. (Since past psychological studies have shown that blood is often thicker than water.)

He found that indeed the more closely siblings resembled each other (based on a third party rating) the more emotionally close they were and the less conflict between them.

"They'll do more favors for each other. They'll interact positively more frequently with one another. In hypothetical situations there is this idea that this individual is my best friend, I'll tell them a secret when I won't tell anyone else. Those are the individuals that they are sharing with. And the siblings that don't look like them precisely the opposite set of patterns."

But ultimately then this concludes that..."facial resemblance is an indicator of kinship even when we believe we already have the kinship problem solved. And we're totally unconscious of it."

—Christie Nicholson