I'm Too Sexy for Your... Virus? Or, Immunity as it Relates to Peacocks

I know something's amiss when my google news alert returns headlines like these:

Why women who lust after Brad Pitt may just fancy his immune system

It's His Immune System That You Actually Want to Sleep With

The key to male sexiness: A powerful immune system?

and my personal favorite

Antibodies, Not Hard Bodies: The Real Reason Women Drool Over Brad Pitt

These snazy headlines are all pointing to a recent paper in Nature Communications. The paper's methodology is pretty simple: They took 74 Latvian men and immunized them against Hepatitis B. Later, they measured the participants' blood for levels of HepB antibody, as well as levels of testosterone. Finally, they showed photographs of each of the participants to a panel of ladies, asking them to rate the subjects' attractiveness. As far as I can tell, Brad Pitt was not involved in this study.

After some analysis they determine a few things -
a) Men with higher levels of testosterone have statistically higher levels of anti-HepB antibodies after vaccination.
b) Men that were rated as more attractive by the female judges had statistically higher levels of testosterone.
c) Men that were rated more attractive had statistically higher levels of anti-HepB antibodies.

This is reported in all the stories above as "Higher testosterone makes your immune system better! Higher testosterone is sexy! Women are attracted to a better immune system!" As you might suspect, it's a good deal more complicated than that.

How is immunity like a peacock's tail?

The most interesting thing about this study (in my humble opinion) is precisely what all those news reports didn't talk about. The whole reason the authors cite for doing this study has to do something called the "immunocompetance handicap hypothesis." The idea goes like this: individuals can provide visual proof of fitness (ability to make strong babies), by ostentatiously displaying something that puts them at a competitive disadvantage. For instance, a male peacock with a huge, colorful tail should have a much harder time hiding from and escaping from predators. If a peacock with such a tail manages to survive despite such a handicap, he must be super fit in other ways, and that fitness is irresistible to peahens.

The immunocompetence handicap hypothesis posits that testosterone works like the peacock's tail. In addition to regulating secondary sex characteristics (broad shoulders, big muscles... you know, sexy things), testosterone is also thought to suppress the immune system. So if a testosterone-filled manly man isn't riddled with illness, he must be super fit in other ways, and that chiseled jaw is just a visual cue to his internal perfection. The reasoning here may seem backwards, but it's on solid theoretical footing.

The trouble is, it doesn't seem like this hypothesis is supported by the data. In this study, men with higher testosterone had stronger immune responses to the vaccine, but I just got through telling you that testosterone is supposed to be a handicap. There's one final wrinkle that I haven't mentioned yet - the stress hormone cortisol. That cortisol suppresses the immune system is well known, and several studies have shown that artificial elevation of testosterone leads to higher levels of cortisol. It seems that the belief that testosterone suppressed the immune system may have been based on an artifact (an effect of the experimental design rather than a true biological phenomenon). In fact, in this new study, the men with the lowest levels of cortisol showed the strongest correlation between antibody response to the vaccine and high testosterone levels.

The authors say that their data shows "support for the stress-linked immunocompetence handicap model of sexual selection," but it seems like they're trying too hard to shoehorn their data into the handicap framework. If high testosterone is related to better immune response, and high testosterone is attractive, you don't need a handicap model at all. To the extent that there is any causal link between attractiveness and immune response (and I don't think the data is that great - see below), it seems that attractiveness is a visual cue of a good immune system, not of some handicap.

I've read the discussion section of this paper a number of times, and I still don't really understand exactly what they're trying to say. Maybe I'm missing something, but while the title of the paper is "Evidence for the stress-linked immunocompetence handicap hypothesis in humans," it seems like the simplistic headlines more accurately describe the data. Carl Zimmer hates it when people write "more research is needed" (because when is more research not needed?), but in this case I think it's valid. There is some (vaguely) suggestive data, and some (maybe) interesting correlations, but nothing like a complete story. I think more research is needed, if only to clarify the point and actually address what the role of this hormone in immune function actually is.

And I still don't know how strong Brad Pitt's immune system is :-(


If you liked this piece, consider submitting it to Open Laboratory 2013 (you can read about Open Lab here). If you didn't like it, let me know why in the comments.


The Data

Because this paper has only one figure and it's pretty self-explanatory, I thought I would show you the data for So you can judge it for yourself. I put it here at the end because I'm not sure how much people care about primary data, but I'd like to include it for those of you that do.

Facial attractiveness is plotted on the vertical axis against a measure of anti-HepB antibodies on the horizontal axis.

Male attractiveness vs Immune response

I'll be honest, I photoshopped out the trend line that was included in the original graph, but seriously, would you conclude from this data that immune response is correlated with attractiveness? There are plenty of unattractive people that showed a great immune response (lower right) and people rated as attractive with low immune response (upper left). It's a bit interesting that the two people with the best immune response were also rated the most attractive (upper right), and all of the people that showed no response to the vaccine were rated unattractive (those are the dots hugging the vertical axis on the left), but even when you see the graph with the trend line, the link doesn't seem that strong (a perfectly horizontal line would indicate no correlation, the steeper the slope, the higher the correlation).

Attractiveness vs immune response (with trendline)

Maybe you're not used to looking at this sort of data, but I think it's especially unconvincing when you compare it to the correlation between testosterone and antibody response:

Testosterone vs Immune response

In my opinion, the best you can conclude from this is that there's something that should be followed up on. There's certainly no causal link, and the correlation isn't even particularly strong.

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This is very interesting, especially for my personal situation. It makes me think, how may it apply for a woman who has been taking testosterone injections for years? I'm a bodybuilder (non-pro) but was very unsuccessful until I started the injections, I continue taking them and feel a lot healthier than back during teenage, but that may be for the reasons that 1) I was slightly overweight & very unathletic and 2) I hated my body and was very depressed & suicidal back then since I always wanted to be (at least) as strong as the males.
I've had contact with a transgender group since I considered for a while to live as a male. There were people with much experience in transitioning between the genders, but I never heard anything about the immune system being affected in neither male-to-female nor female-to-male transgenders who substitute hormones of the target gender. My guess would be that psychological factors play a big role here and thereby, as in myself, a person would be more healthy if they feel better about their body, and if this is achieved by substituting opposite sexual hormones the health effect should be positive. Yes I know, bringing transsexualism into the picture complicates things even more... Thoughts about this?

@ Diane - My thoughts are that this is way outside my area of expertise, and in any case we need to be careful drawing conclusions from anecdotal evidence. However, based on the background reading I did for this post, it seems like the relationship that had been observed between testosterone and the immune system was indirect - the stress induced by the T injections led to increased levels of cortisol and that's what suppressed the immune system.

The negative effects of stress on the immune system are well documented, and I imagine that transgendered people before their transition already have plenty of stress in their lives. Beginning the transition may even end up reducing stress, which you might suspect would improve immune function. I hope that all of my hedging is readily apparent since it's a lot of wild speculation, but something similar may have happened in your case. Depression is also known to have negative effects on the immune system (again - possibly indirectly due to the stress), and finding a way out of the depression may have counteracted any negative effects (if indeed there are any - not proven) of the T injections.

I haven't read the paper yet but while reading your post, I was wondering all along what you mention in the last-to-next paragraph of your comment.

ICHH = secondary sexual traits are honest signals because high testosterone levels cause immune suppression.

The study = results indicate that those with higher testosterone levels have better immune response.

Then: how higher testosterone/good immunity = handicap ???

They could be honest signals of quality all right, but not of the handicap sort.

If you leave out the people who had no immune response to the vaccine (and who knows why they failed to respond), then it's not at all clear that any significant correlation exists in the data between the strength of the immune response and attractiveness. The correlation between testosterone level and attractiveness does hold up if you exclude the non-responders.

By Jung Choi (not verified) on 29 Feb 2012 #permalink

Correction to my previous comment. It's the correlation between testosterone level and vaccine response that still holds up after removing the non-responders.

By Jung Choi (not verified) on 29 Feb 2012 #permalink

@ BDNf - If you do read the paper, please come back and let me know if you caught something I didn't. I seriously read it several times trying to understand, because I was sure I was missing something critical.

@ Jung Choi - Thanks for the response. They actually mention in the paper that some of the correlations drop out of statistical significance if you remove the non-responders, but is that fair to do? I would be super suspicious if they dropped out non-responders that went against their hypothesis, so it seems a bit like a double standard. Then again, I'm not particularly well versed in biostatistics, so I don't know what's permissible.

The attractive / immune graph is clearly a Poulet curve; like a Poisson curve only not.
I imagine some crazy variance in there. (It looks like a chicken, I couldn't resist. )

the chicken comment on the graph made me laugh :D

By sandwiTch (not verified) on 13 Mar 2012 #permalink

This is an interesting post, so I thought I'd share some of my interesting life-experiences and personal data.

If you haven't realized that with the internet there came an explosion of little niche/sectionalized hobbies, sub-hobbies, and how-to blogs, videos, and forums, prepare to be amazed. Otherwise, if you're like me with the internet, this should come as new info but in no way what-so-ever surprising.

I frequent and really enjoy the forums, blogs and vidoes on rsdnation.com, maintained by a company called Real Social Dynamics. RSD is a company that teaches students how to pick up chicks at clubs & bars; mostly the high-end 9's and 10's in Hollywood and New York that probably got there in a movie-star's limo. As you already probably guessed, it's geared towards nerds and geeks that would otherwise have big trouble in this area (myself included ;D).

ANYWAYS, I was drawn to the site because of the scientific model of testing they used. As the saying there goes, it doesn't matter the amount of pickup theory, philosophy, or nutritional supplements you discuss or debate, "It all must be field-tested." I would like to share some data I've observed, collected, and experienced from both myself and other members.

  1.     First off - I just want to point out that Brad Pitt's attractiveness most-likely has to do with his social status (tho I do agree with you, I think Brad Pitt is a very attractive man - and would love to see his immune function in comparison to the data-set of the average American male). Social status and rank has always been important to females and the attractiveness of a man, because it reflects his ability to provide food and well-being for future potential children she might have with him (food, shelter). The pack leader of a hunter-gatherer tribe, whether he got there by either being the strongest or the smartest (maybe he designed a better tool for hunting, now other members must go to him to make more), is always going to have more value because he is going to have other members providing support for him and will have greater capacity to obtain food for more than just himself.
  2.     As far as cortisol goes, I thought you'd be interested to hear that cortisol and stress-levels is big talk on the forums. Everyone who begins a plan to reduce cortisol levels notices immediate improvement in other people's reaction to them as well as their own general feeling of happiness and enjoyment as well. The company founder, Owen Cook (screenname Tyler Durden - he goes by both) centers MOST of his talking points and articles on this, whether it be by using meditation, eating green vegetables, exercise, etc.
  3.     As far as big muscles go - they're not as important as you'd think. Mostly, people find them attractive because they IMPLY the former traits. There's a social implication that big, tan muscles are desirable, and therefore imply a high social status - think about that. You probably won't believe that statement until you've seen a skinny pale guy blow out a big, stupid, over-tanned ape with the hottest girl in the club, while he sits there dumbfounded - too stupid to think of anything to say, while he watches his girl get pulled away from him and man-handled. Physical attractiveness isn't as important to women as it is to men: a woman with large tits and wide hips implies health-levels for producing strong babies and successful childbirth. A man doesn't necessarily need to be big and strong to know how to design the most-effective trap to catch prey; you couldn't discern that by just looking at him, but you could certainly tell by the way the other members of the tribe treat him.

As always, just trying to provide other data points for you to analyze and add pieces to your puzzle. Good luck ;)