The Chemistry of Kissing

by Stephanie Chasteen on July 19, 2009

Since I’m woefully behind in posting on my own blog, I’m grateful to Sarah over at a Schooner of Science who wrote up an interesting article on the Chemistry of Kissing.  I was meaning to write something on this topic for a while, actually, since there was an interesting symposium at the AAAS Meeting in February on the Science of Kissing.  They covered several aspects, including the “genetic sampling” theory described by Sarah, below.  Another researcher at that conference did fMRI scans on people who were in love.  You can hear the Science podcast on that interview here. And a detailed report of the AAAS symposium with all sorts of juicy theories about why we like to smack lips here.

Read more of Sarah’s (aka Captain Skellett’s) posts on her main blog at A Schooner of Science.

Here’s her post:

In the words of Henry Finck, “is not a kiss the very autograph of love?” Well, some kisses are better than others Frinck, and it can be hard to tell who’s gonna be good and who’s not. The one who seems perfect on paper can be absolutely shocking in the lips department, while the bad-for-you going-nowhere person can make you weak at the knees after a mere second of lip action. Why the difference?

If you think about it in terms of biological selection, a kiss is a pretty important thing. I consider it a selection factor, cos sure as hell wouldn’t stay with someone who was a crap kisser, and I bet you wouldn’t either. Now, within my friendship group there’s been quite a bit of cross-dating (or whatever the term is) over the years, and I can tell you that the people I think kiss great do not always get the same ruling from my friends. Some couples have chemistry, and some just don’t.

WHY? What are we tasting on their lips? What in a kiss is so important that it is given a make-or-break status in choosing a mate?

The best theory around is that a kiss gives you information (though taste and smell) about the other persons immune system on a genetic level, in particular the MHC complex. Let me tell you the story.

In the dark and murky depths of chromosome six lies a section of some four million nucleotides, genetic material that encode for MHC’s – major histocompatibility complexes. Histocompatibility being a historical term, as it was first identified as determining which blood type you have – A, B, AB or O. The section of DNA on chromosome six encodes for a whole bunch of different MHC molecules, and the alleles are codominantly expressed – meaning you make both the maternal and paternal products.


Behold MHC molecules, there be the peptide binding cleft and there the transmembrane region that acts like an anchor, yarr!

MHC Class 1 molecules are expressed constitutively in all nucleated cells, while Class 2 molecules are expressed only in special antigen-presenting cells of the immune system, like dendritic cell, macrophages and B cells. There’s also Class 3 products that are secreted instead of membrane-bound, but enough blah-blah, on with the story!



Your body can be a bad neighborhood, so police natural killer cells and other members of the immune system drive by frequently to check the ID of your cells, to see if they are terrorists infected or cancerous. If an MHC protein is visible and is only expressing self-proteins, the cell can live another day.


Now let’s say a cell gets infected by a virus, which pokes in some genes of its own so it can hijack our replicative machinery, much like a pirate commandeers a ship to make booty.


Virus oh noes ensue.



Caught red-handed holding non-self proteins, the cell is told to kill itself quietly (apoptosis), or is ruthlessly killed by the immune system in a dramatic action sequence worthy of Schwarzenegger.

Of course, it’s a little more complicated than this. Instead of just two MHC’s on your surface, you have heaps (it took too long to draw!) The MHC region of the genome is extremely polymorphic, and the goal is to have as many different versions of MHC possible, both in your own DNA and across the species. The more variety there is, the more likely someone out there will have what they need to survive HIV or H1N1 or any of the other freaky viruses that get us worried now and then.

So what would happen if your parents ignored the signs given to them by the almighty kiss, and you don’t have much variety in your MHC’s.



The virus slips past the immune system like a ninja, will replicate and spread, and you’ll get sicklier.

So when we kiss someone, we’re really just saying “Hey, how’s your MHC compared to mine? Ooh… you taste different… MAN our kids will have kick-ass immune systems!” Opposites certainly attract in this case.

How did they discover this? They got men to work out, and then asked women to smell their sweaty shirts and pick which one smelled better, and then they ran genetic tests. Women were more likely to dig the stink of a guy whose MHC was very different to her own.

It’s interesting to note that women on the pill are more likely to choose the WRONG PERSON in these tests, possibly because their body thinks it’s pregnant and it’s a bit late to go choosing a mate based on genetics. This could be a contributing factor to divorce – people hook up when the woman is on the pill, they get married, she stops taking it to become pregnant, and suddenly they lose their chemistry. Something to keep in mind.

So go out there and kiss! Sample the MHC molecules around you, and run your own genetic screening! Albert Einstein himself said “any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.”


Kelly April 26, 2010 at 9:48 pm

Hey great article! I’m 21 and my girlfriend broke up with me a few months ago, but I’m still totally in love with her. So, I’ve been putting a lot of critical thought and research into understanding attraction and love blah blah, so I can understand what happened to us. I’m starting to hear more about this pheromone love compatibility theory and I’m wondering-Can one person be attracted to the other on a chemical level, and the other person not be attracted back? Or is it always an equal attraction? Once again great article!


sciencegeekgirl April 27, 2010 at 12:44 am

Thanks for the comment, Kel. I’ve found science to be somewhat (but only somewhat) reassuring and explanatory in the case of love. The pheromone compatibility theory is, realize, only a theory, so while it could suggest that one person is chemically attracted while the other one isn’t, this idea is still being tested. Plus, humans are complicated test subjects, so it’s hard to say anything sure and true. I’m sorry for your loss — I hope your research helps ease your broken heart!

Tricia May 15, 2010 at 4:34 am

I would say that the reason sometimes people may be “attracted” to someone and the person not be “attracted” back as much would have to go to an emotional level, past the physical chemical science that helps aid in the finding of a mate. I’m not really a science person at all. Yet, how I understand it to be is that this chemical attraction is just one part in many things that help aid us in finding a good mate. We tend to forget that our bodies have natural instincts for survival and I believe that the chemicals that aid in attraction are one of those instincts. Where they can be studied scientifically, emotions and society play a large part in how we choose our mates as well. Those too can be studied but they are so much more unpredictable and harder to analyze than bodily chemicals. Just my uneducated opinion. 🙂 lol

George December 14, 2011 at 6:36 pm

I can actually recall the last time I felt intense chemistry through a kiss, it was 3 years ago, and I haven’t felt anything as intense since. I remember we both felt it, we would literally kiss for hours on end. It was a great feeling, and I nor anyone else I’ve kissed recently has felt such a strong connection through kissing. After I cheated on that person, the chemical feeling felt broken (Like a soda that lost its carbonation) and we went our seperate ways. I’m a believer, and I’m hoping maybe one day I can find that again.

kabelo bethuel lamola October 25, 2012 at 3:50 pm

Just started studying medicine and I must say I’m loving your blogs… Your style of righting is hella fun and entertaining… Keep it up!

Comments on this entry are closed.

Previous post:

Next post: