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George Francis and I have a new exciting study out, the formal and finalized version of George Francis’ 2022 blogpost:

Permanent adipose breasts are unique to humans among primates. We propose that permanent breasts, and a preference for them, are an adaptation for pair bonding and a slow life history strategy. This theory predicts that races with a slower life history strategy will prefer breasts to buttocks. To test our hypothesis, we predict preferences for breasts over buttocks from racial admixture and both national and regional average IQ, which we use as an indicator of differences in life history. Measures of Breast-Buttock preference are constructed for nations and subnational regions using the relative frequencies of internet searches on Google, Pornhub and YouPorn for terms related to breasts versus those related to buttocks. Average IQ correlates with Breast-Buttock preference at the national level (r = .76, p < .001), within regions of American countries (r = .85, p < .001), US States (r = .54, p < .001) and US Metro areas (r = .57, p < .001). Controlling for racial ancestry substantially moderated the effect size of intelligence. We consistently found African ancestry to negatively predict breast preference: across countries (r = −.80, p < .001), within US states (r = −.91, p < .001), and US Metro areas (r = −.91, p < .001). These results replicated when using Spanish language search terms within Spanish-speaking countries, suggesting our findings are not peculiar to the English language. Only for US Metro areas did we find a significant effect size for socioeconomic controls.

Keywords: Sex, Breasts, Intelligence, Life history, Race, Ethnicity

We sought to throw some light on the mystery of human male sexuality preferences for female body features, specifically breasts and butts. Unlike humans, most animals do not have permanently enlarged breasts:

Human females are the only primates with permanent adipose breasts. In other primates, the breasts only develop during pregnancy for the purpose of breastfeeding. There have been many theories for the evolution of perennial breasts in humans including thermoregulation, as a fat source akin to a camel’s hump, and as an adaptation to bipedalism (Pawłowski & Żelaźniewicz, 2021). Currently, however, there is no agreed upon explanation for why humans evolved this trait. For a comprehensive review of the literature see Pawłowski and Żelaźniewicz Pawłowski & Żelaźniewicz (2021). Although the origins of human breasts are uncertain, it is clear that they currently play the role of a secondary sexual characteristic, developing through puberty and being arousing to the male members of the species.

A particular challenge for any sexual explanation of the evolution of breasts is that the cues created by enlarged breasts may have been unattractive to our ancestors. In other primates, enlarged breasts signal being pregnant, or breast feeding infants and possibly having a male partner. During pregnancy it is near impossible to have another conception (Roellig et al., 2011) and females also face temporary infertility during breast feeding, a phenomenon known as “lactational amenorrhea”. A primate attracted to large breasts not only faces the risk of hostility from another male, but also receives few benefits from any resulting copulation. That breasts have evolved as a secondary sexual characteristic, despite possibly signalling sexually unattractive qualities in our ancestors, is a problem we term the “breast paradox”. Pawłowski & Żelaźniewicz (2021, p. 7) refer to this issue as “the key problem” to sexual explanations of the origins of permanent breasts.

In essence, evolutionary considerations posit that men should dislike breasts, and yet they like them. And some like them more than others. I have already been involved in some survey research on this topic, which resulted in a study coauthored with the ever-productive Ed Dutton: Gentle-men prefer faces: Intelligence, life history strategy, and sexual preference for attractive faces over attractive breasts, buttocks, and legs:

We utilised data from the OkCupid dating site to study preferences for body parts in attraction judgements. We found that most people (71%) have a stated preference for faces, with non-Europeans finding other parts more important: lower parts (“butt/legs”; 8%), chest (“chest/breasts”; 3%), and Other (18%). We regressed these preferences on likely causal variables. We found that slower life history speed, female sex, and higher intelligence predict preference for faces and negatively for buttocks/legs/chest, replicating previous research. We further found that most non-European ethnicities have a stronger preference for buttocks and legs, in particular Africans, Hispanics, and Amerindians (Native Americans). We explored how these patterns may help to explain cultural differences in mating display for different body parts and how they may be explicable in evolutionary terms.

In our new study, we sought to examine these relationships at the national and subnational level (e.g. US states) using internet-based data. The key idea here is that men (and some women) who are interested in breasts will search for these on Google or Pornhub (the largest pornography website), and the same is true for men who are interested in butts. While the overall search count may reflect many things — most obviously, the availability of easy internet access and lack of nudity filters — the relative proportion of searches will provide insight into men’s relative preferences for breasts and butts in a given location. Thus, we get metrics such as these:

To make sure the data were valid, we first checked whether the Google searches and the Pornhub data correlated well:

The correlation turns out to be .91, which is astonishingly good for such data. We still see some odd outliers for Asian countries. For example, in Hong Kong, men are among the most interested in breasts according to Pornhub data, but according to Google searches, they are not much different from Europeans.

Looking at the figure, we can also see that poor, mostly African countries tend to be much more interested in butts than breasts. To clarify this relationship, we plotted the data with national intelligence:

Aside from the oddity of some of the Asian countries in the Pornhub dataset, there is quite a strong correlation indicating that countries more interested, collectively, in breasts have higher intelligence.

We also confirmed the African vs. non-African split in the data. True to stereotypes, Africans were much more interested in butts than breasts relative to other humans. This mirrors our findings from the sex worker survey study (also with George Francis):

Sex workers clearly perceive African clients to be more interested in butts. Mind you, these are American sex workers, so they are working with African American men mainly. Our international data thus show that these preferences are consistent across locations. Africans are relatively more interested in butts whether they reside in the USA or in Africa. Thus, this provides another example of the spatial stability of human psychological differences.

But one thing is neat plots — what about the regression results? How do the predictors of breast-butt preference fare when they are competing in a model?

Here we are using the national data, so with a sample size of up to 193 nations, depending on data availability. Overall, we find that national intelligence and African ancestry (proportion of population’s genetic ancestry, as proxied by international migration data) predict preference for breasts and butts in opposite directions. Southeast Asians also seem to have some residual pro-breast proclivities, perhaps due to Indo-European admixture, in part for India at least.

Looking within large countries, we find similar results:

Using estimates of regional average intelligence based on various datasets (see the 2016 Admixture in the Americas meta-analysis), we find that intelligence still predicts preference for breasts over butts. In other words, these differences cannot just be due to national cultural differences, as they exist within countries too.

We can thus also model the data at the sub-national level, yielding a dataset about equally large to the international one. Here we find the same pattern of results, namely, that intelligence predicts preference for breasts, and African ancestry predicts preference for butts. Interestingly, the overall level of development or wealth of a country didn’t predict anything in the presence of these other variables (S = Socioeconomic factor).

Some of the countries used in the above study speak Spanish, not English. So a nice validation of our approach is whether the results still work when we use some of the Spanish terms instead:

We find that it does, though not as strongly. This is perhaps because we didn’t use a sufficient amount or the right Spanish terms. I will let the reader know here, that, yes, we did consult experts in Spanish sexual slang.

There are not as many Africans or African-admixed peoples in the large Spanish speaking countries, but we still see the African ancestry effect here.

Since many people are interested, we also looked at the US subnational data in some detail. First, US states:

Indeed, among US states, the less intelligent ones are populated by people who are more interested in butts.

However, this pattern is almost entirely due to the African proportions of the states, as there is an amazing correlation of .91 between African (Black) population share and the interest in butts over breasts.

However, maybe US states are weird. Many analyses have been done on US states with misleading results due to the “Modifiable areal unit problem” (meaning that state boundaries are somewhat arbitrary and could have been done differently, which might impact results). For this reason, we also explored the metro-level data, that is, data for the largest urban areas:

Yet when using metro-level data, we obtain very similar results.

In fact, the 208 metros produced almost as strong a correlation between breast-butt preference and African proportion as did the state data, r = .82.

One mystery, however, is that among US metro areas, intelligence no longer predicted this preference, while demographics kept predicting it. Also strange was the finding that Asian proportion predicted preference for butts over breasts, opposite of the international data. This suggests some kind of confounding factor, e.g. Asians living in more degenerate or sexually liberated places (choose your own description!) Even the relative amount of development of the metros predicted breast preference. This inconsistency between the international and US metro data is somewhat mysterious. Perhaps one needs to add fixed effects of states, or political ideology (vote shares).

Given this pattern of results, we might speculate on the causes:

We have found intelligence to be associated with Breast-Buttock preference at the aggregate level. This was consistent with our theory that a preference for breasts is indicative of a slow life history strategy, assuming the average intelligence of human groups is an indicator for life history strategy. The finding was robust to controls for GDP and socioeconomic success. We found a significant effect size for the socioeconomic factor only in the US Metro analysis. Given that this results was only found in this analysis, we suggest economic prosperity is unlikely to cause breast or buttock preference.

We suggested that cognitive ability would be correlated with breast preference, but would not cause. Instead, intelligence is likely confounded with life history strategy, which we believe may be causing racial differences in breast preference. Controlling for variables measuring racial ancestry reduced the effect size of intelligence, consistent with our theory. Nevertheless, in many of our analyses cognitive ability still predicted Breast-Buttock preference after controls. This may have been due to the imprecision of our estimates for ancestry making them imperfect controls. Our work leaves open the possibility that intelligence has a causal effect on Breast-Buttock preference.

Whether intelligence causes Breast-Buttock preference may be resolved by studying individuals rather than groups. If brighter individuals do not prefer breasts to buttocks more than their less intelligent compatriots from the same ancestry, this would further strengthen our suggestion that intelligence does not cause these preferences. However, if life history theory is valid for individual differences, we might still expect intelligence to be confounded with life history differences within groups. It may also be possible to test whether individual differences in life history causes variation in Breast-Buttock preference. However, it is debated whether life history theory applies meaningfully within groups (Giudice, 2020; of Menie et al., 2021; Zietsch & Sidari, 2020).

In other words, intelligence levels may simply be proxying the harder-to-measure differences in life history speed. If this is the case, perhaps we should also see historical changes towards short-term mating interest:

If we squint a bit, we can see that the interest in “ass” has eclipsed interest in “breasts” in recent years, insofar as American English books are concerned. Due to the availability of many synonyms for these features, this analysis would require more work. An exciting avenue for future research.

Conclusions

Using international and subnational comparisons across English and Spanish and two different sources of search interest, we find that Africans are more interested in butts and more intelligent people are more interested in breasts. The African butt interest pattern was found when using international data (many native languages), English inside the USA, and Spanish in Latin American countries. Thus, they are hard to explain away, especially when combined with our prior survey research.

The various results don’t really explain the big conundrum of why humans evolved permanently enlarged breasts, though. In fact, it gets more mysterious. For instance, East Asians appear very breast-focused but they have quite small breasts. Somehow evolution did not provide what many Asian men long for (but anime did). It’s hard to see this being the result of some kind of energy constraint, though it is true that Asians are also short, while Asian women prefer taller men.

The rise in butt interest in recent decades could be seen as consistent with moral decay, but the timeline doesn’t fit that well, as the big spike in “ass” in American English books dates back only to the 1990s, not to the 1960s cultural revolution. In fact, mentions of “breasts” start increasing around 1975. Why? I don’t know. We might also wonder why the old books from the 1800s had a relatively higher rate of mentioning these topics than modern books pre-1980.

Our future research will look into the psychological predictors of interest in various body features and other sexual interests. I’m sure the results will be interesting.