We are researchers who work on sexual selection and mate choice. Ask us anything!

Hi Tom, how are you assessing "resemblance" in your study?

Hi! When I started my research, I was originally assessing facial resemblance between couples by scanning their faces and building 3D models that allowed me to make detailed measurements (e.g. head circumference, distance between the eyes, skin pigment density). However, when COVID hit and I was unable to bring people into the lab, I had to use an alternative source of data called the UK Biobank. Now, I’m looking at overall genetic relatedness (i.e. across the entire genome) to assess whether couples are “inbred” to any degree (which may not be as bad as you’d think!). I’m looking at trait-specific similarity (e.g. for height), which can be done at the level of the trait itself (i.e. the phenotype) and at the level of the genes underlying it (this can be done by identifying specific areas of the genome or networks of genes linked to the trait). More technically, I will be taking the particular measurement variable (e.g. height) and using a procedure called regression, which allows you to predict one person’s trait from their partner’s, while controlling for the effects of what we call confounders. For example, it may be the case that height is similar in couples only because they tend to have the same genetic ancestry (a more technical measure of ethnicity. Using regression, you can “control for” the effects of ancestry and effectively look only at height similarity in people of the same ancestral group.

What is your take on humans and monogamy?

Ewan here - thanks for the question! Mating systems vary considerably across animals and explaining this variation is a fundamental task for evolutionary ecologists, so I can’t necessarily give a conclusive answer to this. In animals we normally refer to a number of different systems; “polygyny” (where males mate with multiple females), “polyandry” (where females mate with multiple male), “polygynandry” (where males and females mate multiply with each other), and “monogamy”.

Lots of factors shape what kind of mating strategy is expected to evolve for a given species, with relevant factors including dispersal patterns, parental care, the strength of competition for reproductive opportunities within each of the sexes, sexual conflict - with all of these things depending on and interacting with environmental factors such as resource distribution. So coming up with general statements about this topic is difficult and there are still a lot of unanswered questions! For example, one very controversial question is why would a female mate multiply if mating with one male was sufficient to fertilise all her eggs? That said, it does appear monogamy is relatively rare, although is prevalent in some bird species (often associated with a high level of parental care being required for offspring) and some form of polygamy is more normal. It’s also worth pointing out that the distinction between social and genetic mating systems, for example in many species that appear to practice ‘social monogamy’ (so that pairs live and raise offspring together) are not ‘genetically monogamous’. This occurs because in many socially monogamous species, individuals (of either sex) engage with extra-pair copulations with individuals from other pairs in order to maximise their own fitness. These arguments will likely have applied to ancestral humans, although the ability of biological arguments to explain our modern mating systems is a separate question.

What are the most common ‘similarities’ you see in human pairs?

Tom here! This is a very interesting question. There are several traits that are often strongly correlated in couples across many global populations. The most striking and consistent is genetic ancestry (the genetic basis of people, often described as ethnicity). Beyond this, you tend to find strong correlations for physical, behavioural, and sociocultural traits, including height, intelligence, religiosity, and political orientation. Oftentimes, a correlation in one trait will cause a correlation in another. For example, similarity in height will, on average, lead to similarity in weight. In most cases, the areas of greatest similarity are those traits that define local mating populations (e.g., religious groups, ancestry groups, etc.).

Does this finding account for proximity bias ? Meaning people of same religious beliefs tend to visit and therefore meet in the same place, same with political leaning, personality traits etc, meaning that the real factor is spacial proximity. If we put 2 polar opposites on religious, political, ethnical etc. Parameters in a place where they can frequently meet, and see results would probably to still some factor of success in dating. So basically how do you account for spacial bias in such a framework ?

Tom here - Proximity bias is one of the main reasons for assortative mating. However, the traits that determine proximity (e.g., religious beliefs) may still influence preferences. So, for example, people of the same religion may be more likely to meet and subsequently mate, but they may also actively seek one another out. Decoupling the effects of proximity and preference is challenging. Accounting for proximity can be done by identifying a probable source of bias (what we call stratification), and then controlling for it statistically. In my work, one of the main sources of proximity bias is genetic ancestry (people nearly always cluster in ancestry groups). So if I’m looking at assortative mating for a particular trait (e.g., height), I can control for genetic ancestry and it effectively allows me to look at assortment within ancestral groups.

Is it true people tend to gravitate towards a partner that are like one of thier parents?

Hey! Our response to this Q is related to yours - https://www.reddit.com/r/IAmA/comments/lgw1bs/we_are_researchers_who_work_on_sexual_selection/gmu3pue?utm_source=share&utm_medium=web2x&context=3

Since 2007 and the transition to widespread use of mobile devices (e.g., introduction of the iPhone), what are the biggest changes in mate selection behavior that you have observed?

Tom here. The issue of how technology affects mate choice has many angles and others on this AMA have asked similar questions. For this reason, I’ll give you my broad opinions on how culture and technological development has shaped mating, including recently with the emergence of mobile phones as dating tools.

Historically, technology has helped humans overcome geographical barriers that have historically placed limits on mating. Where once people were limited to mating with those in their local areas, today mate pools are theoretically much larger, even global. This has broken down barriers between once reproductively isolated subpopulations, including ethnic groups, and, as a result, reshaped the genetic makeup of many modern populations.

This is closely related to the creation of dating applications (e.g., Tinder), which have done something relatively similar, allowing theoretical access to a veritable buffet of potential mates while expending little energy. In terms of whether this could affect “dating economics”, perhaps: the circumvention of constraints imposed by time and space may allow for more efficient, lower cost searching for “optimal” mates”. However, because the cost of searching is now so low, it could also have the opposite effect, encouraging more frivolous mating (e.g., attractive with unattractive individuals). In principle, both could co-exist. I find your position intuitively appealing, but I’m not aware of any evidence on this subject (although try looking in psychology and sociology). Another interesting feature of dating applications is that they allow virtual anonymity in the process of mate choice, meaning people can mate outside of restrictive sociocultural niches without necessarily incurring the ire of their cultural group.

Finally, technological modification of the body (e.g., fertility treatments, plastic surgery) has, to a certain degree, allowed individuals to overcome what would once have been biological limitations (e.g., how one’s face looked) constraining mate choice. This has expanded mating opportunities for many people, but also introduced what might be described as “biological deception” (called dishonest signalling in biology) into the act of mate choice: today, and increasingly in the future, what you see is not necessarily what you get, at least from the perspective of passing certain genes of offspring. There are many more aspects to discuss, but hopefully this offers a flavour of the subject as I see it.

Tom, is there a noticeable difference in how much partners resambles each other between a long term relationship and a one night stand? And what about a long term relationship that the partners don't intend to have any offspring? I'm curious about how the "goals" of a relationship affect the selection. Thank you for doing the AMA.

Hi! This is a great question and, as with many, is complicated. In general, couples tend to be more similar in long-term relationships and, as you suggest, the main reason for this may be to improve compatibility for the purposes of reproduction. This is especially important in the case of social compatibility (e.g., working together to raise offspring). On a one-night stand, where there is little prospect of sustained interaction, social compatibility is irrelevant. However, if the goal of the one-night stand is to reproduce, then biological compatibility affecting the probability of conception of offspring survival is critical. In evolutionary history, one-night stands may have been a strategy to reproduce without making a large investment, so compatibility could still matter in this type of mating interaction. So, the answer to your question depends on the kind of trait you’re discussing, but, as a general rule, assortative mating tends to be stronger in long-term relationships.

I once heard someone say, “people are either attracted to someone who looks a lot like them or to someone who looks like their opposite.”. I am short with dark hair and eyes and my husband is 6’7”, with blond hair and blue eyes and he cannot be outside for more than 20 minutes without burning. So at least for me, this seems to be true. Any research to back it up?

Tom here - By and large, it’s generally believed that if similarity does affect mate preference, which is contentious, it predisposes us to prefer those who are alike, but only for certain traits.

It may be possible, for example, that we seek out those who are socially or biologically compatible, or possibly those who are subtly related to us. There aren’t many theoretical reasons to think we’d be attracted to those who are dissimilar, with two exceptions: 1) complementarity: we pursue people who are different because this helps specialisation in relationships (e.g., one person is cold and analytical and makes money, while the other is warm and nurturing and raises children); 2) immunocompetence: it’s been found in many species that having a wide range of genes at the area of the genome called the ‘MHC’ improves our ability to fight novel pathogens. I’m not sure that either of these applies to your particular case. I would speculate that you and your partner’s preferences are not underpinned by any specific predisposition for dissimilarity, and that, to the degree that you differ more than two opposite-sex people plucked at random from the population, it’s a coincidence.

The idea of sexual conflict between genders is interesting. Who typically concedes more in this dynamic?

Ewan here. Thanks for the question! We think that typically most conflicts should be resolved in favour of the sex under stronger selection with regards to the outcome of the conflict, as this sex gains the most by investing most into its trait. However, we also know that aspects of species’ ecology can impact this too, for example in species where relatives compete with each other the outcome of sexual conflict can be strongly modified. Furthermore, ‘chance’ effects of evolution (such as genetic drift) can also lead to outcomes of conflict not expected from looking at selection alone, and similarly the sexes may also be constrained differently in the types of morphology they are able to evolve to overcome each other, which again means you can’t expect selection to always tell you everything.

Further reading: https://www.sciencedirect.com/science/article/pii/S0169534702000046?casa_token=HvjSEyo_ESAAAAAA:qMrS6h0VzWU5uDREmyC6aVIrQ30PHWFVE1Zxo9bfd6pKQeJyfkk08ACxxft-k_uzFWoEdKx9X-0,

https://royalsocietypublishing.org/doi/full/10.1098/rstb.2011.0281

https://www.biorxiv.org/content/10.1101/2020.05.05.080010v2

Is there a growing incel epidemic or have these people just become more visible?

side question: is there a correlation between hyper masculinity and more sexual conflict?

Hey! Our answer here is linked to your side Q - https://www.reddit.com/r/IAmA/comments/lgw1bs/we_are_researchers_who_work_on_sexual_selection/gmtulll?utm_source=share&utm_medium=web2x&context=3

There have been some very interesting papers published about kin selection and how we have affinity to people who resemble ourselves. I'm curious to what thoughts you might have on how that plays into racism in our society. Do you think this might show some connection between racism and mate selection?

Tom here. This is a good question but also a complicated one that involves a lot of speculation.

Kin selection is rarely applied directly to mate choice and usually helps to explain how we direct behaviours such as aggression or altruism. When it is applied to mate choice, it usually takes the following form: we select mates who are SLIGHTLY (slightly to avoid what’s called inbreeding depression) related to us in order to increase genetic representation in offspring, thereby helping the propagation of our genes without actually affecting reproduction directly.

What you’re implying, I think, is that we would select people with similar genetic ancestry in order to achieve these kinds of benefits. In most cases, however, genetic ancestry by itself is a poor predictor of kinship, especially when ancestral groups are large (e.g., continental clusters, such as African or European). The levels of genetic variation within these groups are extraordinary, and superficial similarity of the basis of skin colour and other features would never offer the level of refinement necessary to detect kin. Kin discrimination (i.e., judging how related another person is likely to be) requires much more fine-tuned signals of relatedness (e.g., subtle variation in face shape, scent).

There is another speculative argument for why people would want to mate within kin groups that does not involve kin selection, namely that it prevents the breakdown of groups of genes that have evolved to function together (what we call coadapted gene complexes). However, there is also an argument that mating outside of ancestral groups could be helpful, as it increases genetic diversity at a region of the genome called the 'MHC' (Major Histocompatibility Complex), which is involved in immune function, thereby decreasing susceptibility to specialised pathogens. On balance, other factors play a much more important role in determining the suitability of a mate (biologically speaking) than genetic ancestry, and, where genetic ancestry does drive preferences directly, it is more likely to reflect sociocultural forces, which is another interesting subject.

Dear Tom & Ewan,

Have you used your knowledge to improve your dating profiles on Tinder et al ?

As a result of using your knowledge do you now walk with extra swagger round Imperial ?

Yes

What do you think about duck penises? Asides from the terrifying duck rape and drowning, why on earth would you evolve such a nightmare fuel of a mating processes and sexual organs to boot?

Great question! Ewan here - Two related routes by which we see the evolution of extreme mating behaviours like this are 1) female choice (in the case of duck penises this would be postcopulatory sexual selection) and 2) interlocus sexual conflict. With female choice (specifically cryptic female choice in this case), females in many species are thought to have evolved a reproductive tract that is hard to navigate in order to apply a selection pressure to male sperm so that males with sperm with specific characteristics that are beneficial to the female (e.g. males of higher genetic quality) are more likely to successfully fertilise. This can sometimes lead to escalatory evolution of more extreme sperm (or sperm delivery systems!) and reproductive tract morphology in males and females. With regards to interlocus conflict such as male harmful behaviour of females (e.g. duck drowning), this can be favoured because males gain more reproductive opportunities by harming females if this allows them to mate with them. If the harm issued to females is sufficiently strong they would be expected to evolve counter traits.

Why is colorful plumage considered a desirable trait? As you mention, this lowers animals' ability to avoid predation. So I find it curious to think that a female would think, "Yes, that's a desirable trait. That's what I want my offspring to have -- an easier time attracting prey."

Or does a mating-age bird with colorful plumage indicate that its other characteristics are desirable, as, despite it being easier for prey to spot, it's successfully managed to avoid being caught and killed? Perhaps this would indicate it could be more powerful, agile, aware, etc. than your average, drab-colored fowl?

Great question! In many ways the two most perplexing problems in sexual selection are explaining why females evolve preferences for traits in males (when having a preference is presumably mostly due to searching costs) and correspondingly, why males in turn evolve these costly display traits. As you suggest, our best explanation is that males display traits that are ‘condition dependent’ so that males who are otherwise in good condition (e.g. of good genetic quality) are better able to produce more impressive traits. As such, trait expression correlates positively with male quality, which females then favour because then they will produce offspring with better genes (this is known as ‘good genes’ selection). This in turn can lead to a form of runaway selections as nonrandom mating arising from the female preference trait leads to individuals carrying both genes encoding large preference and display trait values, leading to bigger and bigger values of male traits and female preference. At some point, however, the costs in terms of survival (e.g. from increased predation risk on males carrying the conspicuous display signal, or energetic costs on females to keep searching for males to mate with) imposed from the sexual traits will become so great that the system reaches an equilibrium. An interesting implication of all this is does the evolution of these traits and preferences mean the population is more or less likely to go extinct? On one hand, individuals bear the burden of expressing these costly traits but on the other hand, lower condition/quality individuals (and the genetic variation they carry) are more easily weeded out by selection.

How did you get into this field? Up until right now I didn't even know this was a thing that people researched. Did you always want to do research similar to this, or did you just stumble into it?

Great question!

Tom: When I first began research, I also didn’t know this field/s existed (my work spans various fields, ranging from evolutionary psychology to sociology to population genetics). My first degree was in history and economics, where I became interested in philosophy and human behaviour. However, I decided that to truly understand the foundations of behavioural variation, a biological perspective was crucial. Consequently, I took my master’s in biological anthropology, where I become versed in evolutionary perspectives on human mating and behavioural variation, writing my thesis on the effects of limb proportions on attractiveness judgements. I then took a relatively large leap into population genetics for my PhD, while continuing to focus on human mating behaviour. My current research perspective reflects this diverse experience and, given the fact that human mating behaviour is the product of an extraordinary myriad of social, cultural, environmental, and behavioural forces, I think I’m all the better for it. If you’re interested in studying the biology of human mate choice, therefore, you can use many fields as a springboard, and the research area today is fundamentally interdisciplinary.

Ewan: At school I really enjoyed science and maths and so I studied biology at university. During my holidays I worked as a research assistant in a fruit fly lab where they were doing crazy experiments on nervous systems, behaviour and lifespan. I loved the questions being asked by the group but I was rubbish at labwork (notorious for killing off all the flies by mistake!) and so I moved into theoretical work.

Can any of this be used to select a partner with whom you're more likely to stay with/have a good relationship with, or is it purely more physical attraction if you share similarities in appearance?

Hey! Our answer here might help answer your Q - https://www.reddit.com/r/IAmA/comments/lgw1bs/we_are_researchers_who_work_on_sexual_selection/gmttvpk?utm_source=share&utm_medium=web2x&context=3

Why are we monogamous?

Thanks for our question! You might find this response answers your Q - https://www.reddit.com/r/IAmA/comments/lgw1bs/we_are_researchers_who_work_on_sexual_selection/gmu1mhx?utm_source=share&utm_medium=web2x&context=3

I once had a theory for why couples look similar. A couple communicates often so they observe the others facial expressions and when trying to relate ideas to each other they may mimic expressions subconsciously so over time it may shape their persona, my idea was similar facial muscles, but could just be subtle. Think theirs anything in that?

Tom here. Yes, there is something to this. The process you’re describing is called convergence. As you’d expect, it occurs mainly for social and behavioural traits, as these are the most flexible and therefore easiest to mimic. However, there is some evidence that people do mimic one another’s facial expressions and, over time, become more similar. However, the magnitude of the effect is low, and this certainly can’t account for the majority of similarity in couples. There is a way to test this formally: you take a sample of couples that vary in their relationship duration and see whether this can predict similarity.

What do you think laypeople on Reddit would find most interesting about your research (results or otherwise)?

Tom here - A population-level study in Iceland showed that mates have more, longer-lived offspring when they are 3th or 4th cousins, that offspring themselves produce more children when emulating this mating pattern, and that deviations in either direction produce less favourable outcomes. This was hailed as demonstrating the “biological basis of the third-cousin crush” by one author, but interpreting such associations is challenging. Improvements in fitness associated with inbreeding could reflect sociocultural factors, as in many populations with norms of kin marriage, reproduction between relatives often occurs earlier and benefits from material support.

Is there a correlation between the extinction risk of a population and the reproductive outcomes for the offsprings(or quality of the offsprings, if you will)?

Ewan here - We would expect populations that produce large numbers of offspring in better condition that are also better matched to their environment to be less likely to go extinct. The difficult question for evolutionary ecologists is when are populations able to do this? For instance, does the prevalence of sexual conflict decrease the quality and quantity of offspring being produced in the population?