Another problem with using a list of postdictions to support evolutionary psychology is that you have not shown that it cannot also predict the opposite - as it sometimes can if you choose to tell the story differently.
Premise: Men in tribal societies spend a lot of time hunting, and women more time gathering.
For those people who claim that evolutionary psychology isn't predictive:
These data show that children living with one genetic parent and one stepparent are roughly forty times more likely to be physically abused than children living with both parents.This greater risk rate occurs even when other factors such as poverty and socioeconomic status are controlled. Daly and Wilson concluded that "step-parenthood per se remains the single most powerful risk factor for child abuse that has yet been identified".... Some people, of course, might claim that such findings are "obvious" or that "anyone could have predicted them." Perhaps so. But the fact remains that hundreds of previous studies of child abuse failed to identify step-parents as a risk factor for child abuse until Daly and Wilson approached the problem with an evolutionary lens.
Buss (2008). Evolutionary Psychology: The New Science of Mind (3rd ed.). Prentice Hall. 7. 211-12
And of course, there are many similar examples throughout the book. I literally just opened it to a random page, and found that example.
At some point, the refrain of "Non-predictive! Just-so story! It's a pseudoscience!" starts to look like motivated cognition.
Some lazy googling indicates the phrase "beaten like a red-headed stepchild" was raised to contemporary popular consciousness in 1986's "The wrath", years after Daly and Wilson. Originally it was racist, specifically referring to the way one might treat the result of a wife's earlier dalliances with Irish immigrants.
I'm not sure I trust my intuitions about what an average high school dropout could tell me in the 1970's. But I consider it significant that professionals looked into this question, and did not notice that factor previously, an unlikely result if it was both widely known and actually present.
Much of the commentary here has turned into yet another debate about just how useless and unscientific evolutionary psychology is. (Notice how I cleverly signaled my own viewpoint about the debate. :) But do we really need to have this debate?
It occurs to me that Luke's argument (about the misuse of intuitions by philosophers) really is independent of whether present-day evolutionary psychologists know what they are talking about. All Luke's arguments, if I understand them, really need is that some intuitions are innate and derived from our evolutionary history, some are learned, and that some are generated on the spot by fallible cognitive processes. It really doesn't matter whether an innate intuition arises as an evolutionary adaptation or an evolutionary accident - it still doesn't provide a philosopher what he needs to ground his arguments.
If I am right here, and the validity of evolutionary psychology is irrelevant to Luke's arguments, then lets move on. We can debate whether the validity of evolutionary psychology is irrelevant more generally at some other time.
It is cheating to make predictions that are actually postdictions. To test hypotheses we have to look at questions for which we don't have information and then test those questions by gathering new information. A lot of the "evolutionary psychology" are really "Just So" stories without this kind of hypothesis testing.
A huge amount of science operates on postdictions. That's not 'cheating', it's just not as impressive as Einstein predicting gravitational lensing. Just as in other sciences, including evolutionary biology, evolutionary psychology uses multiple converging lines of evidence to weigh the probabilities of hypotheses (see, e.g., Buss, The Handbook of Evolutionary Psychology, pp. 457-465: PDF).
And in fact, evolutionary psychologists often do make novel predictions and then go out and make the observations that either confirm or disconfirm the hypothesis.
I suspect that those who say evolutionary psychology is nothing but 'just so' stories are not actually keeping up with the science but instead are operating under cached thoughts.
Evolutionary psychology also clearly explains why a significant portion of the human population will be homosexual.
Oh, wait. It does the exact opposite of that. Hmmm. [ETA: I admit kin selection provides a basis for saying that homosexuality might kinda not totally be evidence against the genetic origin of certain traits. It's not the kind of thing that anyone would predict if they hadn't already seen it, and my main point is about ex post rationalization.]
I certainly agree with the general conclusion that natural selection and our specific history as soci...
Haven't read the post yet, but reporting an odd interface behavior:
When I follow links to this post from the "recent posts" sidebar, it works fine
When I follow links to this post from individual comments in the "recent comments" page, I get a "Forbidden" page. The url is different in this case as well: http://lesswrong.com/r/lukeprog-drafts/lw/5bw/your_evolved_intuitions/ rather than http://lesswrong.com/lw/5bw/your_evolved_intuitions/. This is true for both recent and early comments.
Links to other posts from individual commen...
For example, it seems unlikely that we evolved a mechanism that gives us reliable intuitions about the metaphysical possibility or impossibility of zombies.
It's outside the scope of your article, but one thing that I find curious is that people nonetheless do have strong intuitions about philosophical questions for which natural selection wouldn't instill an intuitive answer. Since these intuitions aren't shaped by natural selection, different people, even within the same culture, can have very different intuitions on the same question (e.g., A-theory ...
Heuristics and biases
Human reasoning is subject to a long list of biases. Why did we evolve such faulty thinking processes? Aren't false beliefs bad for survival and reproduction?
Many researchers suggest that while humans are poor at formal logic and Bayesian inference, humans display a kind of 'ecological rationality'.
Other researchers think that our rationality is a direct result of our need to convince others that we're right, and that many of our biases (e.g. confirmation bias) are a direct result of this. http://www.edge.org/documents/archive/edge342.html
Edit - please disregard this post
It seems to me the idea that essentialism evolved for dealing with creatures is a bit too narrow; certainly we don't use it just with creatures, and to the extent it works it does work more broadly than that. I would expect it would more have to do with general factors like speed and scalability (as Eliezer points out in Neural Categories).
The link in "And no, evolutionary psychologists do not only offer 'postdictions'..." is broken.
Even someone who has read Richard Dawkins rant against essentialism
This is kind of a jarring use of the word "rant". Dawkins's argument seems calm and well-reasoned. (Except where he says "Indeed, every one of the series was the child of its neighbor on one side and the parent of its neighbor on the other...". Wouldn't there be at least one animal in the series who was the parent of both its neighbors?)
Heuristics and biases
Human reasoning is subject to a long list of biases. Why did we evolve such faulty thinking processes? Aren't false beliefs bad for survival and reproduction?
Given the nature of evolution and the fact that perfect Bayesianism is computationally intractable, the thing to be explaining is not how bad our reasoning is, but how good it is.
Part of the sequence: Rationality and Philosophy
We have already examined one source of our intuitions: attribute substitution heuristics. Today we examine a second source of our intuitions: biological evolution.
Evolutionary psychology1 has been covered on Less Wrong many times before, but let's review anyway.
Lions walk on four legs and hunt for food. Skunks defend themselves with a spray. Spiders make webs. Each species is shaped by selection pressures, and is different from that of other species.
Certain evolved psychological mechanisms in humans are part of what makes us like each other and not like lions, skunks, and spiders.
These mechanisms evolved to solve specific adaptive problems. It is not an accident that people around the world prefer calorie-rich foods,2 that women around the world prefer men with resources,3 that men around the world prefer women with signs of fertility,4 or that most of us inherently fear snakes and spiders but not cars and electrical outlets.5
An an example of evolutionary psychology at work, consider the 'hunter-gatherer hypothesis' that men evolved psychological mechanisms to aid in hunting, while women evolved psychological mechanisms to aid in gathering.6 This hypothesis leads to a list of bold predictions. If the hypothesis is correct, then:
And as it turns out, all these predictions are correct.7 (And no, evolutionary psychologists do not only offer 'postdictions' or 'just so' stories. Besides, probability theory does not have separate categories for 'predictions' and 'postdictions'.)
Consider the intuition that we have more responsibility for the well-being of our close relatives than for the well-being of distant relatives or strangers. We would expect human evolution to produce exactly such an intuition given Hamilton's rule, which states that the reproductive cost to an agent is less than the genetic relatedness of the recipient to the agent multiplied by the additional reproductive benefit gained by the recipient of the altruistic act.
That's a mouthful, so instead let me illustrate the consequences of Hamilton's rule:
Imagine that you pass by a river and notice that some of your genetic relatives are drowning in a ferocious current. You could jump in the water to save them, but you would pay with your own life. According to Hamilton's rule, selection will favor decision rules that, on average, result in your jumping into the water to save three of your brothers, but not one. You would be predicted not to sacrifice your own life for just one brother, because that would violate Hamilton's rule. Using the logic of Hamilton's rule, evolved decision rules should lead you to sacrifice your own life for five nieces or nephews, but you would have to save nine first cousins before you would sacrifice your own life.8
Hamilton's rule has indeed been observed at work in a wide variety of contexts.9
My intuition that I am more responsible for the well-being of my brother than my cousin, and more responsible for the well-being of my cousin than a stranger, looks like a good candidate for an evolved intuition.
Uneducated people around the world believe that organisms come in discrete packets, and that each species has an 'essence' that produces its form and abilities. The intuitive appeal of this essentialism often trumps the explicitly learned gradualism of biological evolution. Even someone who has read Richard Dawkins argue against essentialism might find himself the very next day stuck in essentialist thinking. Why? Many researchers have suggested that an evolved, intuitive 'folk biology' is responsible.10
These essentialist intuitions emerge early in life across all cultures we have studied.11 For example, children may believe that
...if you remove the insides of a dog, it loses its 'essence' and is no longer really a dog anymore - it can't bark or bite. But if you remove its outsides or change its external appearance so that it doesn't look like a dog, children still believe that it has retained its essential 'dogness.'12
Many researchers think that essentialist intuitions evolved because it's useful for humans to respond to organisms in this way. With essentialist thinking, we can very quickly drop organisms into categories concerning what we can and can't eat, what we can capture, what might capture us, and so on.
Essentialism has had a long-lasting hold on the minds of many philosophers, and greatly influenced their conclusions even after Darwin.
Human reasoning is subject to a long list of biases. Why did we evolve such faulty thinking processes? Aren't false beliefs bad for survival and reproduction?
Many researchers suggest that while humans are poor at formal logic and Bayesian inference, humans display a kind of 'ecological rationality'.13
Over evolutionary time, the human environment has had certain statistical regularities: Rain often followed thunder, violence sometimes followed angry shouts, sex sometimes followed prolonged eye contact, dangerous bites often followed getting too close to a snake, and so on. These statistical regularities are called ecological structure. Ecological rationality consists of evolved mechanisms containing design features that utilize ecological structure to facilitate adaptive problem solving.
The shape and form of cognitive mechanisms, in other words, coordinate with the recurring statistical regularities of the ancestral environments in which humans evolved. We fear snakes and not electrical outlets...
[Moreover], theories of formal logic that are content independent... are exceptionally poor at solving real adaptive problems. The world is full of logically arbitrary relationships: Dung happens to be potentially dangerous to humans, for example, but provides a hospitable home for dung flies. So applying formal logic cannot in principle solve the adaptive problem of avoiding dung. The only thing that can solve it is a content-specific mechanism, one that has been built over evolutionary time to capitalize on the recurring statistical regularities associated with dung as it interacted with our hominid ancestors.14
Our brains may have evolved intuition-generating mechanisms that worked for solving particular adaptive problems in the ancestral environment, but we may not have evolved psychological mechanisms that generate accurate intuitions useful for doing philosophy. For example, it seems unlikely that we evolved a mechanism that gives us reliable intuitions about the metaphysical possibility or impossibility of zombies.
Next post: Intuition and Unconscious Learning
Previous post: How You Make Judgments
1 Recent introductions to the field include: Buss (2011); Workman & Reader (2008); Gaulin & McBurney (2003). It is also worth mentioning one of the major problems with evolutionary psychology. Evolutionary psychologists tend to focus on subjects that are difficult to test because they are uniquely human but also universally human, which is bad for testability (see here and here). For other difficulties, see Problems in Evolutionary Psychology.
2 Birch (1999); Krebs (2009).
3 Buss et al. (1990); Buss & Schmitt (1993); Khallad (2005); Gottschall et al. (2003); Gottschall et al. (2004); Kenrick et al. (1990); Gustavsson & Johnsson (2008); Wiederman (1993); Badahdah & Tiemann (2005); Marlowe (2004); Fisman et al. (2006); Asendorpf et al. (2010); Bokek-Cohen et al. (2007); Pettay et al. (2007).
4 Signs of fertility that men prefer include youth (Buss 1989a; Kenrick & Keefe 1992; Kenrick et al. 1996), clear and smooth skin (Sugiyama 2005; Singh & Bronstad 1997; Fink & Neave 2005; Fink et al. 2008; Ford & Beach 1951; Symons 1995), facial femininity (Gangestad & Scheyd 2005; Schaefer et al. 2006; Rhodes 2006), long legs (Fielding et al. 2008; Sorokowski & Pawlowski 2008; Bertamini & Bennett 2009; Swami et al. 2006), and a low waist-to-hip ratio (Singh 1993, 2000; Singh & Young 1995; Jasienska et al. 2004; Singh & Randall 2007; Connolly et al 2000; Furnham et al 1997). Even men blind from birth prefer a low waist-to-hip ratio (Karremans et al. 2010). Note that standards for beautiful faces emerge before cultural can have much effect (Langlois et al. 1990) and that standards of beauty are relatively consistent across cultures (Cunningham et al. 1995; Cross & Cross 1971; Jackson 1992; Jones 1996; Thakerar & Iwawaki 1979).
5 Buss (2011), pp. 92-94.
6 Buss (2011), p. 85.
7 Evidence cited by prediction number. 1: Hewlett (1991); Lee (1979). 2: Tooby & DeVore (1987). 3: Trivers (1971). 4: Roskraft et al. (2004); Tooby & DeVore (1987). 5: Hawkes (1991); Wiessner (2002). 6: Silverman & Philips (1998); Silverman et al. (2000); Eals & Silverman (1994); Silverman et al. (2007); New et al. (2007); Silverman & Choi (2005); Lippa et al. (2010).
8 Buss (2011), p. 238-239.
9 Buss (2011) calls Hamilton's theory of inclusive fitness (expressed in Hamilton's rule) "the single most important theoretical revision of Darwin's theory of natural selection in the past century" (p. 239). For a review of some of the evidence that supports Hamilton's rule, see Buss (2011), chapter 8.
10 Atran (1998); Berlin (1992); Keil (1995); Medin & Atran (1999).
11 Sperber & Hirschfeld (2004).
12 Buss (2011), p. 73.
13 Tooby & Cosmides (1998). Haselton et al. (2009) say humans are 'adaptively biased,' while Kenrick et al. (2009) say we are 'adaptively rational.'
14 Buss (2011), pp. 396-397.
Asendorpf, Penke, & Back (2010). From dating to mating and relating: Predictors of initial and long-term outcomes of speed dating in a community sample. European Journal of Personality.
Atran (1998). Folk biology and the anthropology of science: Cognitive universals and cultural particulars. Behavioral and Brain Sciences, 21: 547-609.
Badahdah & Tiemann (2005). Mate selection criteria among Muslims living in America. Evolution and Human Behavior, 26: 432-440.
Berlin (1992). Ethnobiological classification. Princeton University Press.
Bertamini & Bennett (2009). The effect of leg length on perceived attractiveness of simplified stimuli. Journal of Social, Evolutionary, and Cultural Psychology, 3: 233-250.
Birch (1999). Development of food preferences. Annual Review of Nutricion, 19: 41-62.
Bokek-Cohen, Peres, & Kanazawa (2007). Rational choice and evolutionary psychology as explanations for mate selectivity. Journal of Social, Evolutionary, and Cultural Psychology, 2: 42-55.
Buss (1989). Sex differences in human mate preferences: Evolutionary hypotheses testing in 37 cultures. Behavioral and Brain Sciences, 12: 1-49.
Buss (2011). Evolutionary Psychology: The New Science of Mind (4th ed.). Prentice Hall.
Buss & Schmitt (1993). Sexual strategies theory: An evolutionary perspective on human mating. Psychological Review, 100: 204-232.
Buss, Abbott, Angleitner, Asherian, Biaggio, et al. (1990). International preferences in selecting mates: A study of 37 cultures. Journal of Cross-Cultural Psychology, 21: 5-47.
Connolly, Mealey, & Slaughter (2000). The development of waist-to-hip ratio preferences. Perspectives in Human Biology, 5: 19-29.
Cross & Cross (1971). Age, sex, race, and the perception of facial beauty. Developmental Psychology, 5: 433-439.
Cunningham, Roberts, Wu, Barbee, & Druen (1995). "Their ideas of beauty are, on the whole, the same as ours": Consistency and variability in the cross-cultural perception of female attractiveness. Journal of Personality and Social Psychology, 68: 261-279.
Eals & Silverman (1994). The hunter-gatherer theory of spatial sex differences: Proximate factors mediating the female advantage in recall of object arrays. Ethology and Sociobiology, 15: 95-105.
Fielding, Scholling, Adab, Cheng, Lao et al. (2008). Are longer legs associated with enhanced fertility in Chinese women? Evolution and Human Behavior, 29: 434-443.
Fink & Neave (2005). The biology of facial beauty. Internal Journal of Cosmetic Science, 27: 317-325.
Fink, Matts, Klingenberg, Kuntze, Weege, & Grammar (2008). Visual attention to variation in female skin color distribution. Journal of Cosmetic Dermatology, 7: 155-161.
Fisman, Iyengar, Kamenica, & Simonson (2006). Gender differences in mate selection: Evidence from a speed dating experiment. The Quarterly Journal of Economics, 121: 673-697.
Ford & Beach (1951). Patterns of Sexual Behavior. Harper & Row.
Furnham, Tan, & McManus (1997). Waist-to-hip ratio and preferences for body shape: A replication and extension. Personality and Individual Differences, 22: 539-549.
Gangestad & Scheyd (2005). The evolution of human physical attractiveness. Annual Review of Anthropology, 34: 523-548.
Gaulin & McBurney (2003). Evolutionary Psychology (2nd ed.) Prentice Hall.
Gottschall, Berkey, Cawson, Drown, Fleischner, et al. (2003). Patterns of characterization in folktales across geographic regions and levels of cultural complexity: Literature as a neglected source of quantitative data. Human Nature, 14: 365-382.
Gottschall, Martin, Quish, & Rea (2004). Sex differences in mate choice criteria are reflected in folktales from around the world and in historical European literature. Evolution and Human Behavior, 25: 102-112.
Gustavsson & Johnsson (2008). Mixed support for sexual selection theories of mate preferences in the Swedish population. Evolutionary Psychology, 6: 454-470.
Haselton , Bryant, Wilke, Frederick, Galperin, Franenhuis, & Moore (2009). Adaptive rationality: An evolutionary perspective on cognitive bias. Social Cognition, 27: 733-763.
Hawkes (1991). Showing off: Tests of another hypothesis about men's foraging goals. Ethology and Sociobiology, 11: 29-54.
Hewlett (1991). Intimate Fathers: The nature and context of Aka pygmy paternal infant care. University of Michigan Press.
Jackson (1992). Physical appearance and gender: Sociobiological and sociocultural perspectives. State University of New York Press.
Jasienska, Ziomkiewicz, Ellison, Lipson, & Thune (2004). Large breasts and narrow waists indicate high reproductive potential in women. Proceedings of the Royal Society of London, B, 271: 1213-1217.
Jones (1996). Physical attractiveness and the theory of sexual selection. University of Michigan Press.
Karremans, Frankenhuis, & Arons (2010). Blind men prefer a low waist-to-hip ratio. Evolution and Human Behavior, 31: 182-186.
Keil (1995). The growth of understandings of natural kinds. In Sperber, Premack, & Premack (eds.), Causal cognition. Clarendon Press.
Kenrick, Sadalla, Groth, & Trost (1990). Evolution, traits, and the stages of human courtship: Qualifying the parental investment model. Journal of Personality, 58: 97-116.
Kenrick, Keefe, Gabrielidis, & Cornelius (1996). Adolescents' age preferences for dating partners: Support for an evolutionary model of life-history strategies. Child Development, 67: 1499-1511.
Kenrick, Griskevicius, Sundie, Li, Li, & Neuberg (2009). Deep rationality: The evolutionary economics of decision making. Social Cognition, 27: 764-785.
Kenrick & Keefe (1992). Age preferences in mates reflect sex differences in reproductive strategies. Behaivoral and Brain Sciences, 15: 75-133.
Khallad (2005). Mate selection in Jordan: Effects of sex, socio-economic status, and culture. Journal of Social and Personal Relationships, 22: 155-168.
Krebs (2009). The gourmet ape: Evolution and human food preferences. American Journal of Clinical Nutrition, 90: 707S-711S.
Langlois, Roggman, & Reiser-Danner (1990). Infants' differential social responses to attractive and unattractive faces. Developmental Psychology, 26: 153-159.
Lee (1979). The !Kung San: Men, women, and working in a foraging society. Cambridge University Press.
Lippa, Collaer, & Peters (2010). Sex differences in mental rotation and line angle judgments are positively associated with gender equality and economic development across 53 nations. Archives of Sexual Behavior, 39: 990-997.
Marlowe (2004). Mate preferences among Hadza hunter-gatherers. Human Nature, 4: 365-376.
Medin & Atran, eds. (1999). Folkbiology. MIT Press.
New, Krasnow, Truxaw, & Gaulin (2007). Spatial adaptations for plant foraging: Women excel and calories count. Proceedings of the Royal Society, B, 274: 2679-2684.
Pettay, Helle, Jokela, & Lummaa (2007). Natural selection on female life-history traits in relation to socio-economic class in pre-industrial human populations. Plos ONE, July: 1-9.
Rhodes (2006). The evolutionary psychology of facial beauty. Annual Review of Psychology, 57: 199-226.
Roskraft, Hagen, Hagen, & Moksnes (2004). Patterns of outdoor recreation activities among Norwegians: An evolutionary approach. Ann. Zool. Fennici, 41: 609-618.
Schaefer, Fink, Grammar, Mitteroecker, Gunz, & Bookstein (2006). Female appearance: Facial and bodily attractiveness as shape. Psychology Science, 48: 187-205.
Silverman & Philips (1998). The evolutionary psychology of spatial sex differences. In Crawford & Krebs (eds.), Handbook of evolutionary psychology (pp. 595-612). Erlbaum.
Silverman & Choi (2005). Locating places. In Buss (ed.), Handbook of evolutionary psychology (pp. 177-199). Wiley.
Silverman, Choi, Mackewn, Fisher, Moro, & Olshanksy (2000). Evolved mechanisms underlying wayfinding: Further studies on the hunter-gatherer theory of spatial sex differences. Evolution and Human Behavior, 21: 201-213.
Silverman, Choi, & Peters (2007). On the universality of sex-related spatial competencies. Archives of Human Sexuality, 36: 261-268.
Singh (1993). Adaptive significance of waist-to-hip ratio and female physical attractiveness. Journal of Personality and Social Psychology, 65: 293-307.
Singh (2000). Waist-to-hip ratio: An indicator of female mate value. International Research Center for Japanese Studies, International Symposium 16: 79-99.
Singh & Randall (2007). Beauty is in the eye of the plastic surgeon: Waist-to-hip ratio (WHR) and women's attractiveness. Personality and Individual Differences, 43: 329-340.
Singh & Bronstad (1997). Sex differences in the anatomical locations of human body scarification and tattooing as a function of pathogen prevalence. Evolution and Human Behavior, 18: 403-416.
Singh & Young (1995). Body weight, waist-to-hip ratio, breasts, and hips: Role in judgments of female attractiveness and desirability for relationships. Ethology and Sociobiology, 16: 483-507.
Sperber & Hirschfeld (2004). The cognitive foundations of cultural stability and diversity. Trends in Cognitive Science, 8: 40-46.
Sorokowski & Pawlowski (2008). Adaptive preferences for leg length in a potential partner. Evolution and Human Behavior, 29: 86-91.
Sugiyama (2005). Physical attractiveness in adaptationist perspective. In Buss (ed.), The handbook of evolutionary psychology (pp. 292-342). Wiley.
Swami, Einon, & Furnham (2006). The leg-to-body ratio as a human aesthetic criterion. Body Image, 3: 317-323.
Symons (1995). Beauty is in the adaptations of the beholder: The evolutionary psychology of human female sexual attractiveness. In Abramson & Pinkerton (eds.), Sexual nature, sexual culture (pp. 80-118). University of Chicago Press.
Thakerar & Iwawaki (1979). Cross-cultural comparisons in interpersonal attraction of females toward males. Journal of Social Psychology, 108: 121-122.
Tooby & Cosmides (1998). Ecological rationality and the multimodular mind: Grounding normative theories in adaptive problems. Unpublished manuscript, University of California, Santa Barbara.
Tooby & DeVore (1987). The reconstruction of hominid behavioral evolution through strategic modeling. In Kinzey (ed.), The evolution of human behavior (pp. 183-237). State University of New York Press.
Trivers (1971). The evolution of reciprocal altruism. The Quarterly Review of Biology, 46: 35-57.
Wiederman (1993). Evolved gender differences in mate preferences: Evidence from personal advertisements. Ethology and Sociobiology, 14: 331-352.
Wiessner (2002). Hunting, healing, and hzaro exchange: A long-term perspective on !Kung large-game hunting. Evolution and Human Behavior, 20: 121-128.
Workman & Reader (2008). Evolutionary Psychology: An Introduction (2nd ed.). Cambridge University Press.