Why Do You Love Your Family?

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Dec 20, 2017


Why Do You Love Your Family?
Why Do You Love Your Family? thumb Why Do You Love Your Family? thumb Why Do You Love Your Family? thumb


  • [MUSIC]
  • Thank you to 23andMe for supporting PBS Digital Studios
  • This is Harrison.
  • Harrison’s favorite hobbies include pooping, peeing, not sleeping, and costing me a fair
  • amount of money.
  • Not exactly the number one qualities you go looking for in a friend.
  • Yet, I knew he was the world’s most perfect human the instant I laid eyes on him, I knew
  • I’d give up everything I have for him, even my life, without hesitation.
  • All of that, even though just a few months ago, we’d never even met.
  • The reason why?
  • Evolution, of course.
  • [MUSIC]
  • Staying alive long enough to have healthy offspring, and getting our traits from one
  • generation to the next is how we win the big-picture game of evolution.
  • It’s kind of the whole point.
  • Some traits, whether it’s being tall, or having stripes, or six fingers on each hand,
  • help us win more than others.
  • If they increase an organism’s chances of surviving and reproducing, then they’ll
  • become more common in the future.
  • This is one of Chuck D’s basic principles in his theory of natural selection.
  • The measure of how successful a trait is at making it to the next generation, is called
  • fitness.
  • A trait that’s more likely to be passed on has higher fitness compared to one that’s
  • less likely to be passed on.
  • What we know today, and what Darwin didn’t know back when he was figuring all this out,
  • is that all these traits from how our bodies are built to how we think are the product
  • of thousands of genes, all interacting with each other and the environment.
  • Knowing that makes you realize we are survival machines.
  • We exist to get our genes into the next generation.
  • Congrats!
  • Now you know the meaning of life.
  • Harrison represents my genes in the next generation.
  • Well, half of them, anyway.
  • He’s my son.
  • Did I not mention that before?
  • What did you think, that I just stole someone’s baby?
  • Now, just for fun, let’s say a pack of velociraptors try to snatch him away.
  • But I step in and sacrifice my life to save his!
  • Which I would totally do.
  • Because he’s awesome.
  • This self-sacrifice is an extreme example of altruism, which basically means your pain
  • for someone else’s gain.
  • Thing is, this kind of risk or ultimate sacrifice–doesn’t immediately make sense when you think of evolution.
  • If I become velociraptor food, my future chances of reproducing are now zero.
  • Because I’m dead.
  • So why would I give up 100% of my genes, the ones in my body, to save just half of them,
  • the ones he carries?
  • It might seem like a really bad exchange on my part, but it actually makes sense.
  • And some pretty basic math can help explain why.
  • Whether an act of altruism is worth it depends on the relationship between the two parties
  • involved.
  • The biologist WD Hamilton actually came up with an equation for this: It factors in the
  • cost to you, C; the benefit to the other individual, B; and how closely related the two of you
  • are, r.
  • Let’s unpack this equation a bit.
  • If the cost to you is less than the benefit gained by the other individual, multiplied
  • by that individual’s relatedness to you, then acting altruistically is worth it in
  • the eyes of evolution.
  • When two individuals aren’t related at all, r will be zero.
  • Let’s say a bus–full of velociraptors–is careening towards a stranger, completely unrelated
  • to me.
  • If I push them out of the way and die myself, I completely lose while the stranger completely
  • wins.
  • They might print something heroic about me in the newspaper, but technically this is
  • not the kind of behavior that evolution rewards.
  • But if the relatedness is greater than 0, things work differently.
  • Let’s say the raptor bus is careening towards my first cousin, and I die saving them.
  • Because my cousin and I have one-eighth of our genes in common--our r is 0.125.
  • In this case, I don’t completely lose.
  • I only *seven-eighths* lose.
  • If I push my sister out of the way of the raptor express?
  • Well, my sister and I have one-half the same genes, so I only *half* lose.
  • Understanding this, biologist JBS Haldane reportedly that he was “prepared to lay
  • down his life for eight cousins or two brothers.”
  • But let’s say I’m given an even more difficult choice.
  • Two busses of velociraptors, one heading towards my brother and the other my child.
  • My brother and Harrison both share 50% of my genes.
  • I’d half-lose either way!
  • How do I decide what to do?
  • Well, I really really hope I never actually have to make this choice, because I love my
  • brother too, but I suspect most parents would save their kid first, and evolution has a
  • good reason for that.
  • Because Harrison is a tiny baby with his whole life ahead of him, and my brother and I are
  • adults, Harrison has the greatest “reproductive potential” among us.
  • That’s a fancy way of saying that Harrison is likely to have more children from this
  • point on than either my brother or I will.
  • This is where we see the influence of genes through generations.
  • Each one of Harrison’s future offspring will share about 25% of my genes.
  • If he has just two kids, I break even on my sacrifice.
  • And if he has more than that, or if any of his kids have kids of their own, I’d actually
  • be ahead.
  • All of these potential grandchildren and great-grandchildren mean sacrificing myself for my child is a
  • better decision for the future of my genes than by letting my child die.
  • Which I would never let happen, okay buddy?
  • Daddy promises.
  • Now we’ve been focusing on the most extreme examples of altruism, giving up your life,
  • but the fact is altruistic acts are fairly common among social animals.
  • Meerkat “guards” keep watch over the colony and raise an alarm if they spot a predator,
  • which puts them at risk.
  • Vampire bats regurgitate blood meals to neighbors that don’t get enough food, and scrub jay
  • chicks hang around the nest after they grow up and help their parents raise their siblings.
  • If the individuals in these groups were completely unrelated, it’d be difficult to explain
  • why they do this.
  • But!
  • If the group members are related to each other, even only distantly, genes that underlie altruistic
  • behaviors like caring for children, protecting members of their group, sharing food, might
  • spread through a population not by upping the odds of an individual's own survival,
  • but by helping an individual’s genes survive in their relatives--a mechanism known as kin
  • selection.
  • Kin selection can help us to understand how complex human behaviors might have evolved.
  • Of course, none of us sit there and run equations in our head to decide if it’s worth helping
  • the people we care about.
  • And we can’t trace an emotion as complex as love or kindness directly to a gene or
  • two.
  • But the way humans behave has been influenced by evolution the same as any other animal.
  • Helping each other helps us survive, and not just in this generation.
  • It ensures some part of us will make it into the next, and the next, and the next.
  • So, why do we care for our family?
  • Why would I, or any other parent, sacrifice everything for our children?
  • It’s evolution.
  • I mean, at least in part.
  • And you can’t spell evolution without love, right?
  • Stay curious.
  • A big thanks to 23andMe for supporting PBS Digital Studios and our show.
  • 23andMe comes from the fact that human DNA is organized into 23 pairs of chromosomes.
  • 23andMe is a personal genetic analysis company that was created to help people understand
  • their DNA.
  • If you and your family each get your 23andMe results you can unwrap how much Neanderthal
  • DNA you each have, and you know, you gotta hurry, because you never know when a bus full
  • of velociraptors might come around the corner.
  • 23andMe has a special holiday offer now through December 26, in case you're looking for a
  • sciencey gift.
  • You can go to 23andMe.com to check out their holiday offer and get kits for your family.
  • Show your support for this show by checking out 23andMe.com/OKAY
  • A big thank you to our special guest Harrison.
  • You guys like his shirt?
  • Let me know in the comments.
  • Maybe we've got another merch idea.
  • How'd you enjoy your YouTube debut?
  • Was it fun?
  • Did you have fun?
  • Leave him a nice comment, and I'll read them to him right before he goes to sleep.
  • Thanks for joining us, we'll see you next time.
  • Stay curious!

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↓↓↓ More info and sources below ↓↓↓

Why do we love people we're related to? Compared to strangers, why do we feel such a deep sense of connection with our family members? Sure, they're nice to us, we take care of each other, and we often live with them, but there has to be a deeper biological reason. That reason, unsurprisingly is evolution. In this video, I explain why taking care of our family, or even dying for them, makes sense in the eyes of evolution.

This video covers more than a half century of evolutionary biology theory and observation, but here's some reading if you'd like to learn more:

Dawkins, Richard. (1976) "The Selfish Gene" http://amzn.to/2Dhd257

Fisher, R. A. (1930). "The Genetical Theory of Natural Selection" http://www.worldcat.org/oclc/880368061

Haldane, J.B.S. (1932). "The Causes of Evolution." London: Longmans, Green & Co.

Hamilton, W. D. (1963). "The evolution of altruistic behavior." The American Naturalist, 97(896), 354-356.

Hamilton, W. D. (1964). "The genetical evolution of social behaviour I." Journal of Theoretical Biology, 7(1), 1-16.


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