Why aren’t we all tall?

There’s a fair amount of social science and anecdata that tall males are more reproductively fit. More precisely, males one to two standard deviations above the norm in height seem to be at the “sweet spot” as an idealized partner (e.g., leading males). And, short men often have fewer children. Short women will pair up with tall men. Tall women will generally not pair up with shorter men. The question then has to be asked: why isn’t natural selection producing a situation where we’re all tall?
As it is, height is a highly heritable trait where there’s a lot of genetic variation present in the population. One hypothesis might be that short(er) people are simply individuals with a higher mutational load. In other words, there’s going to be variation in the load of deleterious alleles from person to person, and one’s value on quantitative traits (intelligence, height) is a reflection of one’s genetic fitness. There are problems with this model, starting with the fact that one you need to tease apart inter-population variation. Also, within families there doesn’t seem to be a correlation between height and intelligence, which you would expect to see if quantitative traits are reflections of variation in mutational load.


So naturally you have to move the possibility of balancing selection. I have suggested in the past that inter-population differences in height may be a function of expected levels of nutritional stress. Short people are smaller, and need to eat less. The same dynamic could produce variation in height within populations as well. But a new paper outlines what I think I think is the most elegant solution (though elegant does not mean right!), Intralocus sexual conflict over human height:

Intralocus sexual conflict (IASC) occurs when a trait under selection in one sex constrains the other sex from achieving its sex-specific fitness optimum. Selection pressures on body size often differ between the sexes across many species, including humans: among men individuals of average height enjoy the highest reproductive success, while shorter women have the highest reproductive success. Given its high heritability, IASC over human height is likely. Using data from sibling pairs from the Wisconsin Longitudinal Study, we present evidence for IASC over height: in shorter sibling pairs (relatively) more reproductive success (number of children) was obtained through the sister than through the brother of the sibling pair. By contrast, in average height sibling pairs most reproductive success was obtained through the brother relative to the sister. In conclusion, we show that IASC over a heritable, sexually dimorphic physical trait (human height) affects Darwinian fitness in a contemporary human population.

There isn’t much theoretical complexity in the paper. They’re looking at a huge data set of individuals from Wisconsin, and they observe that in families where siblings are short the sisters tend to be more fecund, and in families where the siblings are not short the brothers tend to be more fecund. The argument here is that antagonistic sexual selection maintains variation within the population. Some of the media reports suggest some sort of frequency dependent theory in the background; if the population gets too tall or short then males and females of the favored varieties may gain more fitness advantage.
As the authors note over time this sort of dimorphism should fix in a manner where the variation within the population diminishes as sex specific alleles emerge. But this takes a very long time, and may simply be impossible to attain toward equilibrium in the case of a trait with the genetic architecture of height. One would have to imagine modifier genes throwing out their net across the whole genome.
It’s easy to imagine why being tall might entail fitness gains for a male. What’s going on with females? I suspect that on the extreme margin very tall women probably have lower fertility for hormonal reasons. But that doesn’t explain to me why very short women seem to have such high fertility in relation to average height women. One explanation might be that they mature faster, and so enter their peak reproductive years rather early. This might extend their fertile period longer than average height or taller women. In contrast, this isn’t much of a gain for males, who have longer reproductive careers on the tail end.

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August 9th, 2012 Tags: Height
by Razib Khan in Evolution | 19 comments | RSS feed | Trackback >

1. 1.   Sam Says:
   August 9th, 2012 at 5:46 am
   We’re talking about number of children here, not some objective biological fertility score, so you have to take into account perceived attractiveness during mate selection.
   Across most cultures, the one thing that seems to stay (fairly) constant for female attractiveness is age: younger (or, at least, younger looking) women are considered more attractive, and this is particularly true in the United States (where this study was conducted). Indicators of being young include: perky breasts, lighter hair, and being shorter. So shorter women could be more successful because they’re considered more attractive.
   Then, once again because this is an American study, you have to take into account male psychology, and the fragile male ego. Most men do not want to date a woman who is taller than them. Traditionally, men want to feel like the big, strong breadwinner in a relationship, and it’s hard to do that when their girlfriend/wife towers over them. In this way, taller women would be restricted to dating even taller men, while shorter women could date men of any height, and the difference in reproductive success could result from that shrunken dating pool taller women have.
   
2. 2.   Karl Zimmerman Says:
   August 9th, 2012 at 6:54 am
   I’m sorry Razib, but you seem to be being purposefully thick near the end.
   I know I have seen other studies before which suggest that short women are more reproductively fit – that peak fertility happens roughly one standard deviation below average for women. IIRC, this would be around 5’2″ in modern western culture.
   I think the answer seems simple. If tallness is considered attractive in men, and short men are considered less attractive, the converse is also true – tall women are considered more masculine, hence less attractive. At least in modern western society, this works in two ways.
   One is female selection. For a woman who are 5’2″, essentially every man is in her potential dating pool. However, as she increases in height, the potential dating pool decreases unless she becomes less choosy. Women above 5’9′ or so have a potentially small dating pool – and what’s worse, those guys may be highly attracted to short women, meaning they don’t have to settle down with a tall woman at all.
   The other side is male selection, which I think is less an issue, because men tend to be less choosy. I am on the shortish side for a white American man (5’8′) but I dated a few women taller than me by a couple inches. Indeed, I actually found it easier to get dates with tall women than average ones, as they tend to be passed up by men who are intimidated or assume she’s waiting around for someone over six feet. That said, certain aspects of dating – such as holding hands with someone who had the same size hands as me – really skeeved me out, as it made me feel like I was holding hands with a man. I ended up with a preference towards short women anyway. I think it would be the same for most guys – they will be reproductively flexible, but in the end, prefer a shorter woman as a partner if they can find one.
   All things considered, if a tall women is conventionally attractive, she shouldn’t have trouble finding a partner to raise children with. However, I would assume if she’s plain or homely, where her chances would be slimmer to finding a partner anyway, the added burden of height might make it difficult indeed to find a man willing to settle down with her.
   
3. 3.   dcwarrior Says:
   August 9th, 2012 at 7:19 am
   Also, in modern society, doesn’t just about everyone reproduce, such that not only is any particular advantage competing against other countervailing pressures as you note, but also that the “less fit” genomes are not removed from the overall population, but rather are added back to the mix? In other words, the less-preferred short males don’t die and have zero kids, they also get married and their genes get thrown back into the pot.
   Also notice the subtle bias here inherent in the original question – women prefer tall men so tall men are more fit so why isn’t everyone getting taller? Note that if you start it the other way – men prefer medium to short women so medium to short women are more fit so why isn’t everyone getting shorter? – logically it’s just as valid a question but the hypothesis in question is the other way around. It’s acknowledged and dealt with in the text but notice that the question assumes fit men determine the direction height is going.
   
4. 4.   Rhwawn Says:
   August 9th, 2012 at 7:35 am
   > It’s easy to imagine why being tall might entail fitness gains for a male. What’s going on with females? I suspect that on the extreme margin very tall women probably have lower fertility for hormonal reasons.
   My own explanation was a little simpler than Zimmerman: I reason that a tall man is compatible with both short women and tall women – if she’s tall, then they’re equal and it’s OK while if she’s short, then it’s also OK for him to be taller and have to reach down for any interactions. While this is not true for a tall woman: she’s OK if the guy is tall, but if the guy is short (as is much of the male population) then there will be problems as people will notice and joke about it and the guy will feel vaguely ashamed and embarrassed to be with her. So it’s best to be a tall man or a short woman. Short men and tall women will do the worst.
   (Alas, I’ve never seen any hard evidence on whether this is even an accurate description of reality, much less the real reason.)
   
5. 5.   Peter Says:
   August 9th, 2012 at 8:06 am
   1) Short men are less reproductively fit now – but has that always been the case? Fashions come and go on a timescale far too short to show up in evolutionary terms. Height preference could be one such.
   2) Similarly, height is highly heritable now – has that always been the case? Historically I’d have expected height to be much more influenced by environment / nutrition – tallness would be a marker of good status rather than good genes. If we’ve only recently uncovered the genetic variation (by improving nutrition across the board), then there’s simply not been time for selection to act.
   
6. 6.   Razib Khan Says:
   August 9th, 2012 at 8:38 am
   Also, in modern society, doesn’t just about everyone reproduce, such that not only is any particular advantage competing against other countervailing pressures as you note, but also that the “less fit” genomes are not removed from the overall population
   no.
   Historically I’d have expected height to be much more influenced by environment / nutrition – tallness would be a marker of good status rather than good genes
   it’s heritability is very high. if it was only 50%, that’s still VERY heritable (in the west it’s 80 to 90 percent).
   
7. 7.   Miguel Madeira Says:
   August 9th, 2012 at 8:41 am
   Imagine the we all had the double of the height that we have. In these world, it will be considered that we are all tall?
   Of course not – the people less taller than the average will continue to be considered “short” (even if they were taller than the tall people in the “real” world).
   Perhaps this is the reason “why aren’t we all tall” – we really are all tall!
   
8. 8.   Razib Khan Says:
   August 9th, 2012 at 8:44 am
   #7, there are biomechanical and other such limits to height. but over time selection should exhaust variation in the particular direction of a quantitative trait so that all variation should be gone. in other words, the distribution of height/intelligent should be much narrower in absolute terms if directional selection was efficacious over the long term.
   
9. 9.   Neuro-conservative Says:
   August 9th, 2012 at 9:54 am
   So what is the balancing factor for cognition? Brain size/energy expenditure?
   
10. 10.   Jokah Macpherson Says:
    August 9th, 2012 at 10:02 am
    I’ve often brought up intralocus sexual conflict (although I’ve never known what it was called before) when someone was suggesting that certain traits considered attractive in one sex would become more pronounced in the future. I imagine this would apply to other traits besides height as well, such as shoulder size, waist size, etc.
    From my experience men are all over the board on the ideal height of a parter but the average feeling is one of indifference either way.
    
11. 11.   Razib Khan Says:
    August 9th, 2012 at 10:02 am
    #9, probably size. big-ass heads can’t fit through the birth canal. eventually it would be energy and probably structural problems.
    
12. 12.   Razib Khan Says:
    August 9th, 2012 at 10:10 am
    #10, yes, there is a model where high masculine men produce masculine looking daughters, and highly feminine women produce girly men.
    
13. 13.   Anthony Says:
    August 9th, 2012 at 7:50 pm
    There may be something to the male preference for shorter women – while most women will have at least as many children as they want to, barring physical incapacity, some women’s preferences will be dependent on whether they have a husband who will help with raising the child(ren). Taller women will have a harder time finding husbands due to their own and men’s preferences, so one would expect taller women to have slightly fewer children on average. One might further speculate that if height gives an advantage to women in their careers, even if not as strong as for me, that fewer tall women will be willing to single-parent and derail their careers, which would push down the average number of children of tall women a little further.
    This of course assumes that both male and female height is equally inherited from each parent.
    Re #5 and #6 – while it’s true that it’s only recently that almost everyone gets to reach their genetic potential height, the selective pressure for male height would be acting on a random selection of genetically-tall individuals, as those genetically-short men who had the wealth to reach their genetic potential height would still be short. So the selective pressure would still favor genetically-tall people. In the real world, even those genetically-tall men who didn’t reach their full height due to childhood bouts of malnutrition would likely be a little taller than average, as plenty of genetically-short men would have had the same experience, and so despite being stunted, they’d still gain some benefit from their tallness genes.
    
14. 14.   Tom Bri Says:
    August 9th, 2012 at 8:39 pm
    Don’t ignore interactions inside the marriage. As long was we are speculating, consider a marriage with a tall woman/short man. If the woman feels more physically dominant, she’ll able to control the frequency of sex to a greater extent. Less total sex tends to mean fewer babies. Yes, I am assuming that generally a female-dominated couple will have less sex than a male-dominated couple, particularly after a baby or two.
    
15. 15.   Isabel Says:
    August 9th, 2012 at 10:57 pm
    Wasn’t there less dimorphism in our extinct relatives, and even in modern hunter gatherers? What advantage is there for the female to be smaller?
    ” That said, certain aspects of dating – such as holding hands with someone who had the same size hands as me – really skeeved me out, as it made me feel like I was holding hands with a man.”
    I was skeeved out by this comment. What is the advantage of mothers who resemble children anyway? It sounds like a perversion of civilization to me. Like being attracted to women with narrow hips-not smart from an evolutionary perspective.
    
16. 16.   Razib Khan Says:
    August 10th, 2012 at 12:02 am
    Wasn’t there less dimorphism in our extinct relatives, and even in modern hunter gatherers? What advantage is there for the female to be smaller?
    the dimoprhism results are probably not reliable due to small sample size of fossils.
    What is the advantage of mothers who resemble children anyway?
    short women don’t resemble children, for the record. interesting point though about how preferences for size may vary.
    
17. 17.   Hippo Says:
    August 10th, 2012 at 12:04 am
    Fuel supply {ie food) probably has alot to do with it, for a while rencetly, as food was abundant, average height was increasing, but as we cram more and more people into cities, smaller people have the advantage of requiring less fuel (food) and space. This is simply the species adapting to the enviroment. I bet you would find that there are more tall people in rural areas and less in urban centres as a percentage of the population (if you exclude sports which skew the results) as that would simply make sense.
    Notice that many of the largest cities in the world are often populated by the shortest people, I believe this is more than just coincidence.
    
18. 18.   Karl Zimmerman Says:
    August 10th, 2012 at 5:25 am
    17 -
    I think you’re seeing the influence of environment and nutrition on height, coupled with the likelihood of western cities to attract people from developing countries who didn’t reach their genetic height potential in full themselves.
    I find the idea there would be any active selection against the tall in general based upon access to resources alone somewhat preposterous. Keep in mind that tropical forest areas with plenty selected for pygmies, whereas a lot of hunter-gatherers in very seasonal climates in northern Eurasia became very tall indeed. When it comes to extreme isolation, like insular dwarfism, there is of course a bias towards shrinkage, but otherwise there are many ways a population can adapt to a resource crunch, and countervailing pressures (predation, competing groups, etc) could keep the selection on the big an powerful even when the small might otherwise be favored.
    
19. 19.   Grey Says:
    August 10th, 2012 at 6:42 am
    “short women don’t resemble children”
    I’d say it’s almost the opposite. With the same bust and hip size short women look *less* like children i.e. the curvy bits are bursting out all over. Maybe that’s just my impression but geometrically speaking wouldn’t things like waist-hip ratio and the *percieved* size of breasts be influenced by height?
    .
    “There are problems with this model, starting with the fact that one you need to tease apart inter-population variation…They’re looking at a huge data set of individuals from Wisconsin”
    Wouldn’t it make a difference how much choice-based sexual-selection was taking place? In a culture where marriage was through personal choice there would be stronger selection on individual characteristics whereas in cultures where marriages were arranged by the family for other considerations – which i think is most of history in most places since agriculture – individual characteristics may have been secondary.
    Then again if there’s a male-female balancing act on height selection then greater selection may cancel out anyway.
    However another thought on that would be differential sexual selection i.e. in a culture where relatedness and status decided marriage along the male line then there might still be sexual selection of the females i.e. a fixed male choice based on status and family getting to choose one of four available female first cousins and the shortest (and therefore possibly curviest) getting picked thereby selecting on balance for shortness whereas in a more choice based culture the male-female preference sexual selection averages out?
    .
    “Also, within families there doesn’t seem to be a correlation between height and intelligence, which you would expect to see if quantitative traits are reflections of variation in mutational load.”
    What if total mutational load effected both but independently i.e. there’s mutational load related to height and mutational load related to IQ and although they go together they act separately so a family with 10 units of mutational load might randomly have kids with 1-10 points of height load and 1-10 units of IQ load while familes with 4 points of mutational load randomly had kids with 1-4 units of height load and 1-4 units of IQ load. If so then there might be a *higher* correlation between height and IQ among taller families than shorter ones?