Your Evolved Intuitions



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 psychology

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:

  1. Men in modern tribal societies should spend a lot of time hunting, and women more time gathering.
  2. Humans should show a greater tendency toward strong male coalitions than similar species in which males do not hunt much, because strong male coalitions are required to hunt big game.
  3. Because meat from most game comes in quantities larger than a single hunter can consume, and because hunting success is highly variable (one week may be a success, but perhaps not the next week), humans should exhibit food sharing and reciprocal altruism.
  4. We should expect to see a sexual division of labor, due to the different traits conducive for hunting vs. gathering.
  5. Men should exploit status gains to be had from 'showing off' large hunting successes.
  6. Men should have superior cognitive ability to navigate across large distances and perform 3D mental rotation tasks required for throwing spears and similar hunting acts. Women should have superior cognitive ability with spacial location memory and object arrays.

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'.)


Kin loyalty

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.


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'.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.



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