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Do you know why mothers kiss their babies?

Babies crawl around in filthy environments, yet their immune systems aren't fully developed. When a mother kisses her baby, she gets a sample of whatever dirt is on the baby's skin. Her immune system analyzes it for pathogens, and the next day she produces breast milk specially tailored to kill whatever the baby was crawling around in the day before.

Edit: Citaitons:
Raffael Cavallaro's profile photoTony Sidaway's profile photocurtis neveu's profile photoLaura Davidson's profile photo
baby was crawling around with other babies.

pathogen made: to kill babies that will compete for affection from other adults!
Cool, hugs must be improvised blood pressure tests!!
Time to add it to Wikipedia ...
But I thought Wiki was a place were proven facts were recorded.... guess I was wrong all this time ;)
So...tomorrow's comic will be about how easily people are swayed by plausible-sounding ideas, right?
I have also read that the reason mates kiss, often before or during mating, is because the saliva from the male helps the females immune system develop an immunity in a similar way, to the male himself, helping her cope with coming babby.
Adding +Randall Munroe to my "Following" circle has so far been the best value-for-click I got from Google+ :)
You should edit wikipedia too, then it can become true.
Although as far as I can see, there's no mention of kissing. Only skimmed though, so far. :3
one of your citations links to a chevy citation... bwahahaha
chad o
Seems like they would pick up those pathogens anyway, just handling the babies, whether or not they kissed them.

Kissing is kind of a strange thing though. It does seem like something that evolved, but I figured it had something to do with the signals for loving people and loving food getting crossed.'re stating the causality incorrectly...and misleadingly... hence "why do fathers kiss their babies" questions that don't have corresponding answers.
+Christopher Wright "On a scale of one to ten, how delicious is this baby? How about now?"
i know :P (and btw, xkcd is beyond amazing, thanks for awesomeness).
but to phrase it as a question, correctly it would be "Do you know what happens when mothers kiss their babies?", no?
+Andrew Worsnop: I guess there's an argument to be made that they kiss their babies because of what happens, as an evolutionary sort of thing. I don't know if such an argument is actually backed by the research, I haven't read it (despite asking for citations; worst person ever).
+Randall Munroe But you're describing one of the effects of kissing, not why mothers kiss their babies. Or at least, there's no evidence that kissing behaviour evolved because of passive immunology benefits.
yeah... me too actually.. - though i always get weirded out by those "evolutionary sort of thing" arguments. always seemed a bit nebulous to me. Thoughts?
Apparently the same is true about mates. Longer and longer kisses exchanging antibodies as the lead up to conception. Prepping the mother to be stronger. At least, that's what I read a while ago. 
+Örjan Berglin That's not why I kiss my daughter. She's adopted so I already know she's not biologically mine... if that were true wouldn't there be some kind of biological alarm or sense of revulsion going through me? Yet she and I both found it, you know, more of a nice thing. ;-)
How does breastfeeding pass the germs back?
I call BS. If this were true, then licking would be much more effective, and mothers would lick their babies rather than kiss them.
C Weeks
But then most people would lick instead of kiss?
This is too close to the TSA's methods to be true.
But then why do mothers continue kissing their children after breast feeding? Why do fathers kiss their babies?

edit - so it seems like my questions have already been asked. I should more thoroughly first.
Daniel, as long as you what he is trying to , does it really matter if he a verb? He might just be reiterating the fact that he should more thoroughly first.
Talk about spamming a post. Sorry, Randall!
I don't know if it's really true, but it's an awesome story.
When lungs of the fetus is developed enough to survive in the wild, lungs release a protein. Mother's body receives it through amniotic fluid and starts labor.

I've been using this as an example for event based programming.
You won't be able to use them for long.
At the very least, the time frame is off. It usually takes two to six weeks to develop antibodies against novel antigens.
I doubt that's "why" in any meaningful or useful definition of the word, but it is a very cool evolutionary development.
The more likely "why" is because oxytocin promotes affectionate behavior.
OK, as others have said: causality... Also, can you explain my fatherly predisposition to kiss, nuzzle, bite and snuggle my children? I don't know that I do it significantly less than my wife.
+Gokhan Altinoren I wasn't aware the mechanism was known. If it was, it would be easier to induce labor in women who are past-term. Do you have some sort of link or reference for this?
If that applies to people, it implies strongly that it's unwise to induce labor at all if at all possible to avoid it. Very interesting.
Surfactant starts getting produced in the fetal lung at about 20 weeks gestation and peaks at 35 weeks. Premies born after 30 weeks often have enough endogenous surfactant to be able to breathe without assistance. Some papers actually suggest that Surfactant Protein A (which appears to be produced in the uterine lining of the mother in addition to the fetal lung) suppresses labor in humans, and decreased levels in the amniotic fluid coincide with the onset of labor.
That just says they coincide. It doesn't say that one causes the other.
That's why I said "suggest" and not "prove" ;) But the reason why I find it suggestive is because it looks like SP-A inhibits the production of PGF2-alpha in the uterine lining, and PGF2-alpha promotes the induction of labor.
Aren't there are cultures where kissing babies (and even kissing) is not the norm?
+Paul Dyson - no, kissing seems to be part of every culture and goes back tens of thousands of years. One reason couples kiss - the bacteria from a male slowly circulate into the body of a female over time. If a baby suddenly comes into constant contact with the strong bacteria of a male while it has a less developed system, it can be damaged (see above). The baby, in the womb, has constant contact with the father's bacteria and slowly builds up the immunity to the dads bacteria.
+John Weaver Let's say that couples kissing was universal and for the purpose of protecting a baby. Then you would expect the mother to kiss the adults in the household to protect the baby. i.e. the father's other wives, the extended family where the baby will live. Now some cultures have taboos on who can have close contact with who. It would be interesting to see if these taboos followed the pattern of who the baby was allowed to be in contact with. That certainly seems plausible. Unfortunately, this is mere speculation, for if wikipedia is to be believed, couples kissing is not universal, although (to get back to the original post) kissing babies is more widespread.
I think whatever biological and evolutionary advantages kissing may have is, at this point, probably only a very distant second to the fact that on a cultural level humans have decided that kissing is neat.
Jin Kee
Hi! and welcome to Mythbusters....

of course, we now have to explain why politicians kiss babies!
Is it because the politician's plan to breastfeed the babies while the camera's aren't looking? To find out, we're going to start by trying to build a scale model!
Jin Kee
or is it because politicians are social network supernodes and therefore the dirtiest people around?
After we have completed our scale model, we are going to attempt to replicate one of these supernodes, though we already know that the dirtiness comes from politicians being lying scumbags!
Ryan B
Ive been amused by this entire thread, well played.
You've fallen prey to one of the classic blunders of science journalism, Randall. blah blah landwarinasiacakes.

A lot of people writing about science do this, where they find out an effect of some action and that becomes THE. REASON. that people or animals or plants or whatever do that. And then a new effect gets discovered and oh my god! We were totally wrong about the reason!

In fact, nature is a vast and complex thing, and then adding psychology on top of that only makes it more complex. This is an effect of mothers' kissing their babies, obviously, and maybe from an evolutionary standpoint is a reason. But it's also quite obviously not the only reason, and maybe not even the best reason. Lots of things are exchanged while kissing: fluids, pathogens, pheromones, emotions. Probably all of these play a part in the utility of kissing. Plus kissing babies makes them giggle, which is hilarious.
cus they want the moms to think they like babies, so they can get more sex
(to whatever extent "adding psychology on top of" nature has any meaning. Obviously psychology is an emergent result of nature and whatnot.)
True, it's the teleological trap. And not every trait or behavior is an adaptation.
If it works the same way for other mammals then it's one more reason for the mother to clean their babies by licking them.
Might have some bearing of truth...but these days, most people disinfect the shit out of everything, substantially reducing any possible gains for the immune system.

Also: I seldom see mothers kissing the bodily areas of a baby that are the most frequent contact points with the external world.
Oh. Well. I mean, I just have the desire to lick the baby. I don't know, I was pretty much raised by cats. And I lick my husband as a sign of affection. It just seems like the right thing to do!

Also: I often see mothers kissing babies on the bottoms of their feet, their knees, their hands, etc. Wouldn't those be the areas of a baby that are most frequent contact points with the external world...?
I am under the impression that babies kiss their mothers in order to populate their gut with the bacteria required for digestion. Mothers already have gut bacteria, so who knows?
do the mothers wear the gut bacteria on their skin though?
I love this idea! So, after spending 40 weeks compiling the "code", the mother then receives "bug reports" directly from the "product" and distributes patches through lactation.
This more than blows my mind. That's so awesome.
Hey, +Randall Munroe (or anybody), have you found a citation for the "next day" bit? i ask because my understanding for years has been that this production of competing antibodies begins within moments of being exposed to an antigen, which happens in multiple ways including living in the same environment as the child, receiving the antigen via respiration, through skin-to-skin contact, and through mouth-nipple contact. Unfortunately, though, i cannot find the source of this information again. Whatever the time frame, i'd love to find a source that includes it. (i confess, i only skimmed the Jack Newman article, but had read it many times before.)

i'm amused by how many people seemed convinced this just couldn't possibly be true.

Edit: Okay, i found a "soon after oral exposure" in Hanson L. A. (2007). Session 1: Feeding and infant development breast-feeding and immune function. The Proceedings of the Nutrition Society. 66(3), 384-96.
that's crazy, how do ppl figure this stuff out?
Breast milk, its great in all kinds of crazy ways we're just discovering.
it is also a (surprisingly effective) way to find out if a baby has cystic fibrosis. the baby will taste salty, as their sweat is saltier than normal. the breast milk will help the baby with things it swallows, but outside of their gastrointestinal tract, the IgA in breast milk won't protect them.
Do you know why strangers look at strangers?

Strangers see everything as if it were the first time, and therefore they're trying to feel their way through the unchartered terrain. Once a stranger sees another stranger, it's in his or her best interest to observe the other stranger in order to discover what was before unknown.
Do you know why pet owners feed their pets?

When a pet isn't fed, it becomes agitated, and as a pet becomes agitated, the pet owner becomes more and more distressed. Once the pet is fed, then the agitation ceases, and the former stress levels are averted, therefore promoting more feeding in the future.
Do you know why children sharpen their pencils?

When a child is doing their schoolwork, eventually the becomes dull, prompting the child to re-create a sharpened lead so he or she can continue doing the assignment unimpeded by lead loss. This continues until the pencil is "used up", thereby necessitating the need for a new pencil, and the cycle is started again.
Maternal-infant immunity transfer is cool, but I call BS on the kissing part. The original citations only suggest that what the mother is exposed to in the environment (in a general sense) direct the IgA generation; the two follow-up 'sources' about kissing seem to be a poorly translated article about a New Zealand study that seems to suggest the benefit comes more from exposing the baby to bacteria the mother is harboring (basically standard germ theory), and a poorly proofread essay on a site that also discusses "education without school" and "healing without medicine" and cites a book that doesn't actually contain said citation.
Yeah but I'm not so sure we'd bother to analyse, if they weren't so damn cute ;).
But by the time they're crawling around, they should be eating more solid food and relying less on breast milk.
Lots of recent research on the matter...

Breast milk exclusively for 1+ year (until the baby doesn't want it anymore, generally peters out about 2-3 years old) is best. For at least the first 12 months you want breast milk (or formula if you're stuck with it like we are) to be the primary source of calories. They can eat other things but they're not nutritionally significant, just teaching them to chew, taste and generally eat while watching for allergies. They should have nothing but bottle/boob for the first 4 months at least and 6 months is better.

My 8 month old is definitely crawling around but also not old enough to give up the bottle/boob.
The kissing as sampling bit is pure speculation. Please don't do that.
Neat stuff! I had to cut&paste this to my pregnant wife. What, g+ can't share posts via email? Not even gmail?
Add your wife's email address to one of your circles then when you share to that circle she will get an email.
Some people here obviously have never been outside the first world and/or haven't heard of kiss-feeding. <>
It's how our ancestors first ate as infants, it's how infants are fed to this day in many pre-industrical cultures, and it's widely regarded as the origin of kissing.
+John Drummond , yes in your circle she will show up as "sharing by email". From recollection I believe that when you send to that circle you will be asked to tick or untick a box to send email to her and anybody else in the circle who is not a member. If you choose to send, and invitations are still enabled as they are now, she may also receive an invitation to join Google+ (you should tell her she can ignore this).
This is very far fetched, many experiments need to be done to even suggest such an outrageous claim that this might occur. Show me evidence that 1)Kissing alters antigen levels in breast milk. This is an easy experiment to do. Have a mother kiss an agar plate containing a known bacteria present on babies and then see if the breast milk contains antibodies specific for that bacteria 2) the increase in antibodies in the breast milk helps the baby fight infection to that bacteria. This would have to be done in mice because it would never be approved and no mother would agree to infecting their baby to see if their breast milk will save him. Without direct citations to these experiments or similar experiments, your statement is very far fetched.
It's a sweet idea but impossible I'm afraid.
For starters your body takes several days (sometimes longer) to create antibodies. By that time the baby's body would have already reacted to whatever it was in contact with. Barring something really serious, it would be already over.
Secondly, it's extremely difficult for viruses and bacteria to pass through the lips. There are very few things you can catch from a soft kiss.
Our skin, even on the lips, is specifically designed to prevent viruses and bacteria getting in. Otherwise we'd be dead pretty quickly.
They have to come via a mucous membrane. So unless you swab the baby then stick the swab up your nose, your body will not have any way to react to the virus or bacteria.
You may as well rub the baby on your knee and expect an immune response.
Thirdly, if you "ingest" bacteria or viruses (the word used above), they die. Not many bacteria or viruses can survive saliva and stomach acid. Only the few bacteria which specifically evolved for that environment.
So if by some miracle there was a cut on your lip, you then picked up a bacteria from the baby which coincidentally happened to one of the few able to survive (your baby wouldn't be in contact with anything that dangerous), by the time your body reacted enough to supply antibody laden breast milk, your baby would be past the point of needing them.
What's actually happening is that you likely came into contact with viruses and bacteria before your baby did and you created the antibodies then. In fact your baby probably came into contact with the virus/bacteria from you.
The kissing compulsion is just a lovely bonding ritual. It makes you both calm and familiarises each of you with the other's smell.
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