Hi Reddit! We are Tom and Ewan.

Proof - https://twitter.com/ImperialSpark/status/1359085985800351745

This AMA is part of #ImperialLates - free science events for all! Check out this week's programme here.

We are researchers at Imperial College London looking at how we choose our sexual partners and why - both as humans and in the animal kingdom. Our lab focuses on a number of topics across evolutionary biology and genetics, including mate choice in human and non-human primates, the evolution of sexual behaviour, speciation, and conservation genetics in various species

Do you resemble your partner and, if so, why?

Tom here. I work on human mate choice and explore patterns of 'assortative mating'. This is the tendency for mates to resemble one another in heterosexual and homosexual couples. Its occurrence is higher than would be expected under a random mating pattern. I ask why and I also look at the effect of this on reproductive outcomes. At the moment, I’m using a large database (Biobank) of around 500,000 people from the UK to answer two specific questions:

  1. First, I’m using the UK Biobank to test whether assortative mating is stronger in homosexual or heterosexual couples for socioeconomic, physical, and behavioural traits, but also for genetic ancestry (a more precise genetic measurement of what people usually call ethnicity). If there’s a difference, I’ll then try to understand why. This work is part of a wider series of projects being undertaken in my lab, headed by Vincent Savolainen, on the evolution of homosexuality in non-human primates.
  2. Second, I’m using genetic data from the UK Biobank to identify what we call “trios”, which are groups of three people containing two parents and their biological offspring. I’ll then look at whether the strength of assortative mating predicts reproductive outcomes for offspring, such as health in infancy and adulthood, or problems during pregnancy. The idea here is that matching for certain traits might increase parental genetic compatibility, ultimately helping offspring in various ways.

One of the overarching goals of these projects, especially the second one, is to explore ways in which natural selection might have affected assortative mating, offering some, albeit tentative, indication about whether we should expect the behaviour to occur in normal behaviour.

Sexual selection and evolutionary suicide

Ewan here. I’m an evolutionary geneticist and theoretician, and I build models that explore how choice in mates affects how populations evolve. We know that choice in mating partners affects the distribution of traits or characteristics in a population, so the evolutionary trajectories of many species are directly impacted by sexual behaviour. I use mathematical models to study this.

In particular, I look at the consequences of mate choice on genetic variation and population viability. For example, certain mating preferences in one sex can lead to the evolution of expensive traits in the other (such as colourful ornaments – think of a peacock’s tail). These traits can increase an individual’s mating success but at the expense of some other characteristic (such as the ability to avoid predation), which may lead to increased death rate and even extinction.

One class of sexual behaviours that have a particularly strong effect on population viability are those that generate ‘sexual conflict’. Because of their different reproductive biologies, males and females often favour very different strategies to maximise their fitness (ability to produce offspring). Sexual conflict arises when strategies evolve that are favourable in one sex but harmful to the other.

For example, in many species, males evolve behaviours which are harmful to females, such as harassment, or killing offspring sired by other males. These traits benefit males by coercing females into mating with them, thus increasing their own reproductive output, but simultaneously diminish that of the females they interact with. Clearly these kinds of behaviours have the potential to significantly reduce population viability because they decrease the total number of offspring that females can produce, and in extreme cases it is thought that male harm can become great enough to drive extinction – a case of ‘evolutionary suicide’!

However, the consequences of sexual conflict in populations can be very complex, as the existence of harming behaviours in males can favour the evolution of counter-adaptations in females, often called ‘resistance traits’, which mitigate the effects of male traits. In fact, one fascinating outcome of this can be a sexual “arms race”, as each sex sequentially evolves more and more extreme behaviours in order to overcome those evolving in the other! 

Using mathematical models, I study how sexual conflict shapes which behaviours will be favoured by natural selection and the consequences of this for population demography, such as extinction risk.


Ask us anything! We’ll be answering your questions live 4-6PM UK time / 11AM-1PM Eastern time on Wednesday 10th February.

Further information:

- Research on animal homosexuality and the bisexual advantage - https://www.imperial.ac.uk/news/190987/scientists-explore-evolution-animal-homosexuality/

- Overturning ‘Darwin’s Paradox’ - https://www.imperial.ac.uk/stories/overturning-darwins-paradox/

- Ewan Flintham’s Twitter page - u/EwanFlintham

- Tom Versluys’s academic homepage - https://www.imperial.ac.uk/people/t.versluys18

Comments: 794 • Responses: 21  • Date: 

realfutbolisbetter320 karma

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

ImperialCollege229 karma

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.

model_citiz3n310 karma

What is your take on humans and monogamy?

ImperialCollege327 karma

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.

TheMidgetCanadian241 karma

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

ImperialCollege452 karma

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.).

InnoSang169 karma

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 ?

ImperialCollege167 karma

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.

Why_am_I_adulting197 karma

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

badchad65133 karma

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?

ImperialCollege149 karma

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.

veybi94 karma

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.

ImperialCollege85 karma

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.

Purplesparkleglitter70 karma

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?

ImperialCollege103 karma

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.

JohnnyFootballHero55 karma

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

ImperialCollege42 karma

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,



_-obscurity-_33 karma

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

uneekElf18 karma

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

Lefthandfury33 karma

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?

ImperialCollege75 karma

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.

MavisNN312 karma

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 ?

ImperialCollege47 karma


Bigluce11 karma

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?

ImperialCollege11 karma

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.

Sciencetist10 karma

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?

ImperialCollege8 karma

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.

Fallenangel24935 karma

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?

ImperialCollege7 karma

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.

DeadlyDancingDuck5 karma

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?

LeviJean3 karma

Why are we monogamous?

Davemblover693 karma

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?

ImperialCollege7 karma

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.

IDontFuckingThinkSo2 karma

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

ImperialCollege3 karma

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.

uneekElf2 karma

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)?

ImperialCollege5 karma

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?