The Six Lines of Evidence for Innateness
Test whether a human difference is evolved or merely learned before believing either story
- Difficulty
- Moderate
- Time to result
- ~weeks to results
- Steps
- 6
- Confidence
- 82%
Rather than proving innateness with one killer study, Stewart-Williams stacks six independent lines of evidence and treats their convergence as the real signal. A trait that (1) appears very early in life, (2) emerges despite culture actively suppressing it, (3) has stayed stable across a century of measurement, (4) tracks prenatal hormone exposure, (5) recurs across nearly every culture, and (6) shows up in other species under the same selection pressures is very unlikely to be pure socialization. Any one line is rebuttable; all six pointing the same direction is not. The output is a calibrated judgment — 'there is an innate contribution here' — that still leaves room for culture to shape the exact size and expression of the difference.
Origin
Developed by evolutionary psychologist Steve Stewart-Williams and laid out in his book on sex differences as the six lines of evidence he uses to argue that a given difference has an innate basis, not just a socialized one.
Core principles
- 01No single line of evidence seals the deal; convergence is what makes the case.
- 02Socialization is real, but it is not the only cause of a difference.
- 03A difference that appears despite culture is stronger evidence than one that merely matches culture.
- 04Cross-species and cross-cultural patterns expose what culture alone cannot explain.
- 05Innate does not mean fixed, good, or permissible — it only means partly unlearned.
How to run it
- 1
Check early developmental appearance
Ask how early in life the difference shows up. Differences in aggression and risk-taking appear almost as soon as children can move around and take risks, before much socialization could plausibly act.
Pro tip The earlier and more automatic the difference, the harder it is to pin on learning.
Watch out Early appearance alone is not proof — infants can be socialized fast, so treat it as one line, not the verdict.
- 2
Look for the difference emerging despite culture
Check whether the difference persists even when culture pushes against it. Most cultures try to suppress young male aggression, yet it still rises at puberty — evidence it is not simply culturally manufactured.
Pro tip 'Despite culture' is stronger evidence than 'because of culture', because socialization would predict the opposite.
- 3
Test persistence across time
See whether the difference has stayed stable across decades of measurement. Sex differences in mate and career-related interests were evident from the early 1900s and held steady through the 20th century despite strong pressure to change them.
Pro tip Persistence through eras of deliberate social change is a costly test for the pure-socialization hypothesis.
- 4
Look for hormonal correlates
Check whether the trait tracks hormone exposure, especially prenatally. Girls exposed to high prenatal testosterone (congenital adrenal hyperplasia) later show more male-typical interests, independent of sex.
Pro tip Within-sex variation tied to hormones is powerful because it holds sex constant while the trait still moves.
- 5
Test cross-cultural universality
Ask whether the difference recurs across most or all cultures. If it were purely cultural you would expect wide variation, yet men out-perpetrate women on homicide in every nation with data.
Pro tip One study on career interests replicated in 80 of 80 nations — that level of uniformity is rare in social science.
Watch out Universality shows an innate push, not a fixed magnitude — culture still tunes the exact size.
- 6
Check for the pattern in other species
See whether the same difference appears in species subject to the same evolutionary selection pressures. Stewart-Williams finds this the most persuasive single line — it points straight to the ultimate cause.
Pro tip Weight species that share our selection pressures (e.g. parental-investment structure), not just any animal.
In the wild
Applied to aggression, the framework converges hard. It appears early (boys hit the ER more from risk-taking as soon as they can move), despite culture (societies actively try to suppress male violence yet it climbs at puberty), persists over time, tracks prenatal testosterone, is cross-culturally universal (90%+ of homicides are perpetrated by men in every nation with data), and mirrors other species (male chimps commit 92% of chimpicides, near-identical to humans). No single line is decisive, but six pointing the same way make socialization-only implausible.
→ A calibrated conclusion that the aggression difference has a real innate contribution, while culture still shapes its size and expression.
Common mistakes
Resting the whole case on one line
Stewart-Williams is explicit that none of the six lines seals the deal alone. Citing only early appearance or only cross-cultural data invites easy rebuttal; the strength is in convergence.
Reading 'innate' as fixed, good, or justified
An innate contribution says nothing about whether a trait is desirable or permissible. Sliding from 'evolved' to 'therefore fine' is the naturalistic fallacy and corrupts the whole analysis.
Is it for you?
Best for
Anyone weighing a nature-versus-nurture claim who wants to reason from converging evidence instead of a single anecdote or ideology.
Not ideal for
Someone seeking a single decisive test — this framework deliberately refuses to rest on any one line of evidence.
From the transcript
“in the book I lay out six different lines of evidence for an innate contribution to any given difference”
“None of them are perfect, none of them by themselves seal the deal... they all point in the same direction”
“often these sex differences appear despite culture, rather than just because of culture”
From the episode
The Uncomfortable Science Of Sex Differences - Steve Stewart-Williams - #1120
Williams