Why is 'Grudger' an evolutionary stable strategy?

Why is 'Grudger' an evolutionary stable strategy?

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I am currently reading 'The Selfish Gene' by Richard Dawkins, which I am sure many here have read. The topic are evolutionary stable strategies (ESS) regarding cooperation.

I apologise for the long question. If you are already familiar with the topic and Dawkins' model of Cheat, Sucker and Grudger: my question is, how can Grudger be an ESS if it could be invaded both by Suckers (because they have no disadvantage against Grudger) and Cheats (because a Cheat minority is unlikely to meet the same Grudger twice, turning Grudger into Sucker effectively)?

More detailed:

The model

Near the end of chapter 10 (p 185 in my version), Dawkins uses a model of birds who clean each other of parasites, therefore helping in survival (as cleaning themselves they cannot reach every spot of their body). He defines three different behaviours for the model:

  • Sucker - birds who indiscriminately help and clean other birds
  • Cheat - birds who let others help them but never do so themselves
  • Grudger - birds who help others and remember who they helped. If the same bird does not help them later (reciprocate), they will not help that bird again.

Claim: Cheat and Grudger are ESS

He claims that both Cheat and Grudger in themselves are ESS - that is, if all birds behave this way, none of the other behaviours can develop because they will be immediately penalised by lower chances of reproducing.

The part that makes sense: Suckers is not an ESS, Cheat is

Sucker is of course not an ESS. If all birds were Suckers, any Cheat that developed would have a huge reproductive advantage and Cheat genes would overtake the population.

Being an ESS makes sense for Cheat. If all birds cheat, nobody will ever be helping each other. A minority of Suckers would be spending all their time helping and not getting anything in return, Cheats have the advantage and Suckers die out again. Grudger would be unlikely to meet a Cheat who they helped before again, so they too will spend all their time helping and die out again.

The part that confuses: Grudger is an ESS?

But Dawkins also claims that Grudger is an ESS, and he seems very confident in that. Now I don't consider myself enough of a smartypants to claim that he's wrong, but I don't understand how Grudger can be an ESS. If all birds behave in this way, and for any reason some Sucker developed - the Sucker would have no disadvantage. All birds would still always be helping each other, so nothing would stop the Suckers from propagating equally well as the Grudgers, invading the gene pool. That's already the ESS broken, but even further, the presence of Suckers would mean that if Cheats came up, they would have a realistic chance of surviving - Grudgers would shun them after having helped once, but if the number of Suckers is large enough, Cheats will have an advantage.

Moreover, back to the initial setting of Grudgers only - if a Cheat developed, he would be unlikely to meet the same Grudger twice, receiving the benefit all the time but never paying the cost. He would have an advantage and spread Cheat genes.

The problem

I'm not familiar enough with how these kinds of models are calculated in order to state chances that Cheats will take over completely, but however I think of it Grudger does not seem to be an ESS to me.

Does anyone have an explanation why Dawkins is so sure that it is? Seeing as in nature we do see patterns like Sucker and Grudger all the time, I must be missing something important here.

Unfortunately it is not necessary to invoke group selection to answer this question. This is one of the reasons that Dawkins likes this discussion so much - he does not believe in group selection and so the discussion in SG does not invoke group selection. ESSs are described in the book as the product of direct competition or interaction between genes.

ESSs in this case, can be described in terms of game theory. In the famous Prisoner's Dilemma experiment, Grudger is similar to tit for tat, which 'won' the competition in the original Axelrod contest.

To see how this works you make a simple win/loss game matrix:

G NG G win 1 win 3 NG lose 3 lose 1

if you are groomed you win 1, if you are groomed, but you don't have to groom - even better win 3 (say) If you groom but are not groomed, lose 3 If neither of you are groomed, you both lose 1

one might argue the exact proportions, but the point is that getting some thing for nothing is better than reciprocating, and getting nothing for your efforts and time are a loss, because you could have been getting groomed by someone else. As you can see cheaters end up in the top row all the time. grudgers end up along the diagonal, and once in a while in the lower left, Suckers get stuck in the lower left a lot whenever there is a cheater around.

now run this encounter over and over. A behavior which scores negative the more times you run is not stable - they are going to disappear from the population, at least if this disadvantage is real

It has more than one stable outcomes in populations, a population that is full of Grudgers will all groom each other as before you know everyone, you assume they will reciprocate. Everyone wins!

Any invading Cheaters will quickly be at a disadvantage, in that they will not be groomed more than N times where N is the number of grudgers in the community. Note that there is an equilibrium here - the Cheaters may exist in a small number - when N is large enough for a cheater to get enough grooming to make a 'living'.

Suckers can also exist within a population of grudgers, but a population of Suckers where Cheaters show up are quickly sucked dry by the cheaters over several generations where you tally up 'points' and give more, healthier offspring to high scorers. They are not ESS stable.

Cheaters are also stable - nobody ever wins, but they don't lose big either and any invading grudgers can't get groomed.

In an infinite, well mixed population with single pairwise encounters, Grudger is indeed not an ESS. In fact, as you correctly note, in such a model the Grudger and Sucker strategies are indistiguishable, as the probability of anyone encountering the same individual twice is zero.

To make it possible for the Grudger strategy to survive against invasion by Cheaters, we must somehow extend the model to allow pairs of individuals to meet more than once. Some ways to achieve this include:

  • Finite population size: if there are n individuals and they each participate on average in m encounters during their lifetime (or during the average time for which their memory persists), then each of them will encounter every other individual m / (n−1) times on average.

  • Viscous population: this is a general term for populations that are not well mixed. For example, if individuals live on a spatially extensive landscape, have limited movement rates and interact only with nearby individuals, then two individuals that meet once have a higher probability of meeting again due to spatial proximity.

  • Iterated encounters: in these types of models, pairs of individuals are assumed to interact with each others some (fixed or random) number of times before parting and finding new partners to interact with. In this way, repeat encounters can be included even in infinite, well mixed population models. While this may be a reasonable approximation in some cases (e.g. for models of spousal cooperation in serially monogamous species), frankly the main reason for studying such models seems to be that they're mathematically simpler than finite or viscous populations.

Not entirely coincidentally, many of these mechanisms can also permit the survival of pure Sucker or altruist strategies against invasion by Cheaters through group and/or kin selection (or more general forms of assortment).

Ps. Even with these mechanisms, Grudger will never be a strict ESS anyway, since in any population consisting of only Grudgers and Suckers both have the same payoff.

An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans

Dizygotic twinning, the simultaneous birth of siblings when multiple ova are released, is an evolutionary paradox. Twin-bearing mothers often have elevated fitness, but despite twinning being heritable, twin births occur only at low frequencies in human populations. We resolve this paradox by showing that twinning and non-twinning are not competing strategies instead, dizygotic twinning is the outcome of an adaptive conditional ovulatory strategy of switching from single to double ovulation with increasing age. This conditional strategy, when coupled with the well-known decline in fertility as women age, maximizes reproductive success and explains the increase and subsequent decrease in the twinning rate with maternal age that is observed across human populations. We show that the most successful ovulatory strategy would be to always double ovulate as an insurance against early fetal loss, but to never bear twins. This finding supports the hypothesis that twinning is a by-product of selection for double ovulation rather than selection for twinning.

Kin selection

When JBS Haldane remarked ‘I will jump into the river to save two brothers or eight cousins’, he anticipated what became later known as Hamilton's rule (1). The ingenious idea is that natural selection can favor cooperation if the donor and the recipient of an altruistic act are genetic relatives. More precisely, Hamilton's rule states that the coefficient of relatedness, r, must exceed the cost-to-benefit ratio of the altruistic act:

Relatedness is defined as the probability of sharing a gene. The probability that two brothers share the same gene by descent is 1/2, while the same probability for cousins is 1/8. Hamilton's theory became widely known as ‘kin selection’ or ‘inclusive fitness’(2-7). When evaluating the fitness of the behavior induced by a certain gene it is important to include the behavior's effect on kin who might carry the same gene. Therefore, the 𠆎xtended phenotype’ of cooperative behavior is the consequence of ‘selfish genes’ (8, 9).

Difficulties with applying Evolutionarily. Stable Strategy (ESS) methodology and terminology to alternative mating behaviors (in which some males in local populations adopt strikingly different, often non-competitive, behavioral patterns) are reviewed. Definitions for “tactic” (behavioral phenotype) and “strategy” (evolved set of rules for expressing tactics) are given. Inconsistent and incorrect applications of “mixed,” “pure,” and “conditional” ESSs are discussed.

Cases of condition-dependent alternative mating tactics are reviewed. Because most alternative behaviors are condition dependent, neither their population-wide nor individual fitness contributions are expected to equal the fitness contributions of “primary” tactics. Individuals should, however, switch tactics at “equal fitness points.” A particular conditional tactic will persist when its maintenance cost (genetic or physiological) is less than its fitness contribution. In only exceptional cases are the fitness contributions of tactics expected to be equal: 1) genetic polymorphisms, 2) stochastic “mixed” ESSs, 3) frequency-dependent choice and, 4) arbitrary assessment. Although alternative tactics may occur in cases of genetic polymorphism or genetic equipotence, most mating tactics probably occur when continuous heritable variation in underlying conditional strategy exists. Selection for genetically influenced “roles” (genetic background) may also uncover apparent heritability.


We thank L. Chao, X. Fu, X. He, J.-D. Huang, A. Murray, M. Vergassola, C.-I. Wu and G. Zhao for discussions, and Y. Wu and H. Zhou for assistance with bioinformatic analyses. C.L., W.L. and D.L. acknowledge financial support by the Major Research Plan of the National Natural Science Foundation of China (91731302), National Key Research and Development Program of China (2018YFA0902700), Strategic Priority Research Program (XDB29050501), Key Research Program (KFZD-SW-216) of Chinese Academy of Sciences, and Shenzhen Grants (JCYJ20170818164139781, KQTD2015033117210153, Engineering Laboratory [2016]1194). T.H. and J.C. acknowledge support from the NIH through grant R01GM95903.

Why Did Humans Evolve The Ability To Feel Broken-Hearted?

Is there an evolutionary advantage to getting heartbroken? originally appeared on Quora: the knowledge sharing network where compelling questions are answered by people with unique insights.

Answer by Suzanne Sadedin, evolutionary biologist, on Quora:

To my knowledge, there's no evidence heartbreak is adaptive in itself, although I can think of some ways in which it might be.

Heartbreak is a consequence of emotional bonding. Emotional bonding is adaptive for a semi-socially monogamous species like humans because we work well in cooperative long term relationships, particularly caring for offspring together.

Several neurotransmitters (e.g. oxytocin, vasopressin, serotonin and dopamine) change levels during interactions with loved ones. These changes make us feel happy and crave the company of that person, which is adaptive because it promotes the relationship. Some of these effects are addictive in the same way drugs are. When the relationship ends, the neurotransmitters abruptly return to baseline, causing the misery of withdrawal.

Arguably, the potential for heartbreak encourages honest and stable relationships because people fear losing their relationships and dealing with heartbroken (and potentially spiteful) ex-partners. Honest and stable relationships may well be adaptive, and might in turn lead to more successful tribes and societies. But that's speculation, and selection at the level of tribes is most likely too weak to cause heartbreak to evolve.

This question originally appeared on Quora. Ask a question, get a great answer. Learn from experts and access insider knowledge. You can follow Quora on Twitter, Facebook, and Google+.

How superstitions spread

Ancient Roman leaders once made decisions about important events, such as when to hold elections or where to build new cities, based on the presence or flight patterns of birds. Builders often omit the thirteenth floor from their floor plans, and many pedestrians go well out of their way to avoid walking under a ladder.

While it’s widely recognized that superstitions like these are not rational, many persist, guiding the behavior of large groups of people even today.

In a new analysis driven by game theory, two theoretical biologists devised a model that shows how superstitious beliefs can become established in a society’s social norms. Their work, which appears in Proceedings of the National Academy of Sciences, demonstrates how groups of individuals, each starting with distinct belief systems, can evolve a coordinated set of behaviors that are enforced by a set of consistent social norms.

“What’s interesting here is that we show that, beginning in a system where no one has any particular belief system, a set of beliefs can emerge, and from those, a set of coordinated behaviors,” says Erol Akçay, an assistant professor of biology at Penn.

“Slowly, these actors accumulate superstitions,” adds Bryce Morsky, a postdoctoral researcher. “They may say, ‘Ok, well I believe that when I observe this event I should behave this way because another person will behave that way,’ and over time, if they have success in using that kind of a strategy, the superstitions catch on and can become evolutionarily stable.”

Morsky and Akçay’s work is an application of game theory, which attempts to predict how people will interact and make decisions in a social setting. They specifically considered what are known as correlated equilibria, scenarios in which all actors are given correlated signals that dictate their response to any given situation.

“A classic example is a traffic light,” says Akçay. “If two people are approaching an intersection, one will get a ‘stop’ signal and one will get a ‘go’ signal and everybody knows that. It’s rational for both parties to obey the light.”

The signal, in this case the traffic light, is known as a correlating device, or more evocatively, a “choreographer.” But the Penn team wanted to know what would happen if there was no choreographer. If people could pay attention to a variety of other signals that could direct their actions, and their beliefs were transmitted according to the success of their actions, would coordinated behaviors arise? In other words, can evolution act as a “blind choreographer?”

“What if a cyclist is riding toward an intersection, and instead of a traffic light they see a cat,” Akçay says. “The cat is irrelevant to the intersection, but maybe the person decides that if they see a black cat, that means they should stop, or that maybe that means the approaching cyclist is going to stop.”

Despite the color of a cat having no bearing on the likelihood of an approaching cyclist stopping or going, sometimes this kind of conditional strategy might result in a higher payoff to the cyclist—if it is correlated with superstitions of other cyclists.

“Sometimes it may be rational to hold these irrational beliefs,” Morsky notes.

In their model, Morsky and Akçay assume that individuals are rational, in that they do not follow a norm blindly, but only do so when their beliefs make it seem beneficial. They change their beliefs by imitating successful people’s beliefs. This creates an evolutionary dynamic where the norms “compete” against one another, rising and falling in prevalence through the group. This evolutionary process eventually leads to the formation of new social norms.

Morsky and Akçay showed that the evolutionarily stable norms, those that cannot be replaced by others, have to be consistent, meaning that they successfully coordinate individual behavior even in the absence of an external “choreographer. “

They found that these evolutionarily stable norms, in both prescribing how an actor should behave and also describing that actor’s expectations of how others should behave, create a consistent belief system that helps coordinate the overall behavior of many actors, even if that coordination is not being directed by any outside choreographer.

To further explore their findings, the researchers hope to engage in social experiments to see whether individuals might start devising their own superstitions or beliefs when none are provided.

“What I like about this work,” says Morsky, “is that these beliefs are made-up superstitions, but they become real because everybody actually follows them, so you create this social reality. I’m really interesting in testing that further.”

Support for the research came from the Defense Advanced Research Projects Agency (Grant D17AC00005) and Army Research Office (Grant W911NF-12-R-0012-03)

Erol Akçay is an assistant professor in the School of Arts and Sciences’ Department of Biology at the University of Pennsylvania.

Bryce Morsky is a postdoctoral researcher in the School of Arts and Sciences’ Department of Biology who works with Akçay.

Dynamics and stability of evolutionary optimal strategies in duopoly

In this paper, we analyse the company behaviour in duopoly taking into account the most common strategies, including dominant, reactive, cooperative and tit-for-tat strategies, since they account for most of the decisions made by companies. Dominant, reactive and cooperative strategies may lead to different outcomes, such as Stackelberg, Cournot, Cartel, Cartel with one cheater and Perfect competition equilibria, while the tit-for-tat strategy may lead to Cartel, Cournot or Perfect competition equilibria due to its retribution nature. However, we argue that these outcomes are mainly valid in the short-run since in the long-run the companies learn and adapt to the behavioural pattern of their peer, which leads them to evolve to a new way of thinking and strategic planning taking into account the long-run effects. Due to the long-term perspective, the outcome of the implementation of various strategies in duopoly may not be efficiently solved using simple game theory analysis. For that purpose, we propose the utilization of more complex analysis, an evolutionary game theory, which is based on the adaption of the company to the behaviour of other players in a duopoly. When the probability of choices is applied, new fitness equations are obtained which show the changing tendencies towards the companies’ strategies that ensure payoffs above the average, in the long run, using a replicator dynamics concept. We analyse several scenarios, in which players choose among two, three or four strategies. Our results indicate that the long-run equilibrium and preferred options are significantly altered depending on the starting set of strategies.

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Axelrod's tournaments¶

In 1980, Robert Axelrod (a political scientist) invited submissions to a computer tournament version of an iterated prisoners dilemma. This was described in a 1980 paper titled "Effective Choice in the Prisoner's Dilemma".

First tournament¶

  • 15 strategies submitted.
  • Round robin tournament with 200 stages including a 16th player who played uniformly randomly.
  • Some very complicated strategies, including for example a strategy that used a $chi^2$ test to try and identify strategies that were acting randomly. You can read more about this tournament here:
  • The winner (average score) was in fact a very simple strategy: Tit For Tat. This strategy starts by cooperating and then repeats the opponents previous move.

The fact that Tit For Tat won garnered a lot of research (still ongoing) as it showed a mathematical model of how cooperative behaviour can emerge in complex situations (why are we nice to each other?).

Why Feminists Must Understand Evolution

I am a feminist but I am not here to offer opinions, nor to enter into an intra-feminist debate. For all their various ideological differences, all feminists basically advocate the same things: for women and men to have the same rights and duties as citizens, and for women and men to enjoy the same freedom to decide what to do or not to do with their lives. I am here to present empirical evidence which ought to interest feminists, and which can help to explain human behaviour.

It is my goal to explain why the causes of male and female difference are not merely cultural or the product of patriarchal indoctrination. Separate athletic competitions and distinct medical disciplines of gynaecology and urology testify to the most obvious biological differences between men and women. But the scientific method − a co-operative, critical, and self-correcting process which has midwifed huge technological and medical advances − can also help us to understand more subtle differences between the sexes in interests and aspirations. And it is understanding what we really are that will make us free.

The study of other animals has produced significant advances in our understanding of human biology. We have been able to understand how our neurons function from the study of sea slugs and squid we know how our embryos develop from the study of sea urchins, toads, and quails we understand how the circulatory system works, and how to repair it when things go wrong, because we have studied the circulatory systems of pigs and dogs. Human physiology textbooks are full of data obtained by studying other animals, and the application of this knowledge has allowed us all to live longer and better lives. But the study of animal models also indicates that male and female differences are not only physical but also behavioural, and that they are a product of our common evolutionary history.

All human beings have something in common: we are offspring. We are the result of individuals being able to reproduce, who in turn were the progeny of other offspring who have managed to do the same. This chain is theoretically traceable along a lineage of individuals who reproduced successfully, all the way back to our origins. Those who did not reproduce did not leave a copy of themselves, and so no longer exist. (A more meticulous explanation of the functioning of evolution through natural selection and genetic drift, or what is known as synthetic theory, can be found on the UC Berkeley website. 1 )

Sexual selection is an important driver of evolution.

Accordingly, each living being is potentially reproductively effective, because it is the offspring of reproductively effective parents. But sexual reproduction depends not only on the capacity to produce viable and fertile offspring, but also on finding a suitable reproductive mate. To qualify, this must be an individual of the opposite sex or, more precisely, someone who can provide gametes of the kind usually produced by the other sex. One of the sexes produces big, static gametes (eggs, which are relatively ‘expensive’ to produce) and the other produces small, rapidly moving gametes (sperm, which are somewhat ‘cheaper’). In many species, the sex with the ‘expensive’ gametes (the female) takes care of many other costly facets related to reproduction. For instance, a female turtle will cross an ocean to lay her eggs on the beach, and a female spider will regurgitate her own innards so that her offspring can feed, literally eating her to death. (Compared to examples like these, waking up at 3am to breastfeed the baby does not sound too exacting.)

Of course, the onus of expenditure does not fall on the female in all species, but whichever sex bears the greater cost of, and makes the greater investment in, child-bearing and -rearing will always be more selective when choosing a mate. After all, it is they who will bear the heavier consequences of a mistake (for example, failing to leave descendants or leaving only a few in return for their investment). So the underlying mechanisms guiding mate selection are subject to great pressures to be effective, and these inevitably bear on behavioural differences between the sexes. These pressures have produced powerful discriminatory abilities which make us selective, even petty, and lead us to subject all possible reproductive partners to constant evaluation. Historically, this arrangement has been an effective and successful reproductive strategy, given that the descendants are alive to make copies of themselves today.

The reproductive cost is undeniably greater for the human female, and the morphological differences between the sexes imply differences in what has been selected for in each sex to make us more effective breeders. But it is also important to understand how the physiological and anatomical differences between male and female reproductive strategies impact our behaviour.

Female baboon nursing her offspring.

Among feminists, there exists a pervasive tendency to believe that animals and humans play different roles in the world, and are subject to different rules. Some ascribe this difference to ‘culture’ or ‘intelligence,’ while others ascribe it to ‘society.’ However, this alleged distinction between humans and other animals does not stand up well under scrutiny.

Certainly, our cultural dimension affects the way we reproduce, but we cannot modify it much. This is because the mechanisms we have evolved to choose a mate and to reproduce are a product of our biology, passed down a long lineage of successful breeders. It is therefore reasonable to expect humans to be a typical species in this respect, just as we are in the examples offered earlier (neuron and heart function, embryonic development, and so on). Evolutionary biology predicts that each individual will try to pursue the best strategy to contribute genetically to future generations, and to produce offspring who will, in turn, produce offspring of their own.

But this strategy will be different for men and women, due to their distinct reproductive functions. The efficacy of the strategies pursued by our ancestors has determined something as simple and fundamental as the very fact that we exist at all. These strategies, then, are a fundamental part of us, even if social and cultural relations modulate them. It is only a slight exaggeration to say that from the moment we awake until the moment we go to bed, most of our actions have the ultimate purpose of leaving a progeny (or keeping that progeny alive, at least until it is old enough to produce descendants of its own).

This process manifests itself differently in males and females, and produces different behaviours. Women, by virtue of our greater reproductive investment, are generally very selective. Men, then, are only truly selective if they consider they will have to make a strong investment of time and resources in a relationship. 2 As a consequence, men and women all over the world, across cultures, tend to look consistently for different things in the opposite sex (though, logically, they have common preferences as well). Furthermore, each sex emphasizes very different aspects of their own personality and physique in the attempt to attract a mate. 3 4 5 6 7 This, in turn, makes competition among men very different to competition among women the former is generally more obvious 8 and the latter is more subtle (and more pernicious, in my opinion). 9 10 11 12 13 14 15

These differences manifest as the differences we observe in our daily lives: from the toys we prefer when we are small to the products we consume when we are adults from the tendency to be the object of bullying or its perpetrator to the likelihood of causing a traffic accident from the posture we adopt when we sit in the underground to the importance we attach to career status.

Intrasexual competition among women can manifest as disapproval of clothing or behaviour that signals sexual availability.

These behaviours occur without us being too conscious of why we do what we do (other than the fact we feel like doing one thing or another). But we do not need to know that we are implementing a reproductive strategy in order to carry it out. 16 17 We simply feel like behaving in a certain way, without interrogating the true cause of our predispositions. (For example, when we crave a hamburger, it is seldom with the conscious awareness that the consumption of many fats and carbohydrates in a few grams of food is an efficient strategy for obtaining energy.)

The fact that men and women are different in these respects does not preclude feminists from striving for completely equal rights between the sexes. However, it is important to understand how things really are if we are to try to modify them, and history provides us with examples of the hazards associated with pursuing an insufficiently tested theory. Convinced that the differences between male and female brains were social, a medical researcher and his team persuaded the parents of a baby boy who had lost his penis in a botched circumcision to raise him as a girl. 18 In spite of a course of hormone injections and the parents’ best efforts to deceive their child, in the end they had no choice but to concede defeat (with terrible consequences for all involved).

But some feminists would prefer to doubt the applicability of evolutionary biology to the human species. They believe that equality of behaviour in the sexes would exist in nature, but culture generates our inter-sexual differences (for examples see Chapter 1 in A Mind of Her Own). 19 20 Apparently, contradicting this line of thought means that one is adopting a ‘biological determinist’ position, undesirable because it is provides a justification for systemic inequality and gendered violence. However, coming to this conclusion requires a significant degree of scientific and historical blindness.

Resistance to acknowledging biological differences in behaviour arises from a fear of the consequences of tying these differences to three clearly erroneous assumptions: 1), that what is natural is good, 2) that what is natural is correct, and 3), that what is biologically-based is impossible to modify.

If all natural things were good, then companies making orthodontic braces would have gone bankrupt long ago, we might die of an intestinal infection at the age of 19, and we would have as many children (or almost as many) as we have orgasms. The same naturalistic fallacy pertains to the justification of behaviours based on a natural tendency to carry them out. It might be natural to have sex with 13-year-olds who are already sexually mature, or to simply take what we find along our way as we see fit, or to use other species cruelly for our personal benefit. And yet, most of us do not do these things, nor do we excuse those who might. That a form of behaviour has its basis in biology does nothing to recommend it. Cultural norms are agreements about conduct and ethics, and they need not be justified with reference to what is and is not natural. Finally, with regard to whether all phenomena with a basis in biology are immutable, we can refute such a statement with reference to the improper and infrequent behaviours itemised above, or by observing that guide dogs refrain from marking their territory at every corner.

A commonly held and erroneous assumption is that what is biologically based is impossible to modify.

If our common goal is to encourage reciprocal respect for other individuals, in spite of average differences in group proclivities, then that goal cannot be well served by ignoring the basis for such differences. The imposition of respect may work in certain cases, but it does not seem to have made much impact on the number of deaths women face at the hands of men, which has remained remarkably stable year-on-year. We can more productively fight gender problems if we acknowledge naturally occurring differences upon which we can work, instead of imposing rules that only increase misunderstanding, allow fallacies to proliferate, and instrumentalise fear as a motor for change.

Some feminist authors insist that it is injurious to consider sex-based differences in the fight against gender inequality. 21 But asking people to ignore the existence of biologically grounded sex-based differences only makes the disparities produced by those differences more difficult to understand and address. Other feminists argue that the very fact of being female authorises them to opine on the motivations of women with absolute certainty. But this is simply to generalise on the basis of one’s own particular example without the benefit of systematic evaluation.

It is better to generate our opinions and judgements based on observations that conform as closely as possible to objective reality, because our goals are political and we want them to affect each and every one of us. It is therefore imperative that we understand the nature of the reality we are trying to change, and the reasons why attempts to encourage complete parity of the sexes in all walks of life through social policy have not yet been successful and have, in some cases, led to the widening of disparities. Political action cannot be founded on opinions about how we would like the world to be (of which there is one for every person). It must instead be built on the foundation of our best understanding of natural reality as it is.

The good news is that information has never been more freely available. If we make the effort to learn a little English and master basic statistics, each one of us can draw her own conclusions based on the work others have already completed. What’s more, those who are not persuaded by this work can try to disprove it using the very same tools of investigation and analysis. Others may simply choose to discard measurement and reason, electing instead to behave much like those who reject the efficacy of vaccines, or insist that humans never went to the moon. But such behaviour does not allow us to build anything it is only good for yelling into the wind and promoting norms which have nothing to do with reality, and which therefore can contribute nothing to the process of effecting meaningful change.

We may prefer to believe that the differences leading us to behave in sexist ways stem from culture, and not from a lack of it. But, by so doing, we will continue to try to impose norms not commonly shared, which will only aggravate the differences between us, making the society we co-inhabit increasingly hostile and founded upon ever more artificial human relations. Ideological ideas accepted a priori by many feminists, such as “language is sexist and changing it will reduce differences”, have not been properly evaluated as instruments for achieving equality. This matters because, in order to change the world, we must first study what we are, and why we behave as we do.

If the goal is not the pursuit of knowledge and understanding, but the promotion of dogma which insists that only socialisation generates sexism, I am afraid the glass ceiling will remain above women, the number of femicides will remain unchanged, and our efforts to improve society will be a perpetual source of disappointment and frustration. We must strive for a synthesis of the scientific knowledge of human behaviour and the political objectives of feminism. It is up to us to keep an open mind so we can better understand one another, the societies we have built, and the world we share. By these means alone, can we create the conditions necessary for real equality.

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[2] Buss D, Schmitt D. Sexual Strategies Theory: An Evolutionary Perspective on Human Mating. Psychological review. 1993 Available from:

[3] Buss D. Sex Differences in Human Mate Preferences: Evolutionary Hypotheses Tested in 37 Cultures. Behavioral and Brain Sciences. 1989Available from: S0140525X00023992

[4] Schmitt D. Sociosexuality from Argentina to Zimbabwe: A 48-Nation Study of Sex, Culture, and Strategies of Human Mating. Behavioral and Brain Sciences. 2005 Available from: http://journals.cambridge. org/abstract_S0140525X05000051

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By Marta Iglesias

Marta Iglesias is a predoctoral researcher in the Champalimaud Neuroscience Programme, Lisbon. Her research is focused on how evolution shapes brains and behavior in competitive contexts such as mate selection and aggression.

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