Heritability of IQ
No study of the causes of intelligence has failed to find a substantial heritability.
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- Twin vs Adoption
| Relationship | IQ Correlation (%) |
|---|---|
| The same person tested twice | 87 |
| Identical twins reared together | 86 |
| Identical twins reared apart | 76 |
| Fraternal twins reared together | 55 |
| Biological siblings | 47 |
| Parents and children living together | 40 |
| Parents and children living apart | 31 |
| Adopted children living together | 0 |
| Unrelated people living apart | 0 |
| ![[Genome#^cc80af | ^cc80af]] |
Heritability of Personality
- Twin vs Adoption (mainly based on Big Five: OCEAN)
| Relationship | Personality Trait Correlation |
|---|---|
| Identical twins reared together | ~50% |
| Identical twins reared apart | ~40–50% |
| Fraternal twins reared together | ~20–25% |
| Siblings (non-twin) reared together | ~20% |
| Adoptive siblings | ~0–5% |
| Unrelated individuals | ~0% |
- Conclusion: parental ≠ sole influence, also genetic and influence of peers
- Twin and adoption studies consistently find
- Genetics explains ~30–60% of the variance in traits like intelligence, temperament, or risk of mental illness
- Peers, unique experiences, and random variation matter a lot
- Parenting often has surprisingly small effects on long-term personality ((personality difference undetectable in identical twins separated at birth raised in dramatically dissimilar environments, ‘parents influence less than we think’ well supported)
people get their personalities from their genes and from their peers, not from their parents.
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- Parental influence
- Attachment styles, trust and safety, language development, early cognitive stimulation
- Moral, religious, political, cultural values
- Parenting as environment in GxE (stress response), and severe neglect, abuse, or trauma can override genetic predispositions and lead to dysfunction
- While long-term personality traits may be mostly genetically driven
- Private & public lives/personalities may be separated
But children do not seem to let the home environment influence their personality outside the home, nor to let it influence their personality in later life as an adult.
Link to original - Studies mistook correlation (parent style-child) for causation, without controlling for heritability
Because virtually no studies had controlled for heritability, there was no proof of causation at all in any study. Not even lip service was being paid to this omission: correlation was being routinely presented as causation. Yet in each case, from behaviour genetics studies, there was new, strong evidence against what Rich Harris calls ‘the nurture assumption’. Studies of the divorce rate of twins, for example, reveal that genetics accounts for about half of the variation in divorce rate, non-shared environmental factors for another half and shared home environment for nothing at all.
Link to original - Behaviorally, children also affect parents
it is now clear that children probably have more non-genetic effect on parents than vice versa. As I argued in the chapter on chromosomes X and Y, it used to be conventional wisdom that distant fathers and over-protective mothers turn sons gay. It is now considered much more likely to be the reverse: perceiving that a son is not fully interested in masculine concerns, the father retreats; the mother compensates by being overprotective, Likewise, it is true that autistic children often have cold mothers; but this is an effect, not a cause: the mother, exhausted and dispirited by years of unrewarding attempts to break through to an autistic child, eventually gives up trying.
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Equality amplifies heritability
If you accept the criticism that these studies mildly exaggerate heritability because they are of families from a single social class, then it follows that heritability will be greater in an egalitarian society than an unequal one. Indeed, the definition of the perfect meritocracy, ironically, is a society in which people’s achievements depend on their genes because their environments are equal. We are fast approaching such a state with respect to height:
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遗传性是指：用来衡量一个性状（如身高、智力、体重）在一个人群中，有多少比例的个体差异可以归因于基因差异，而非环境因素
它并不是说这个性状完全由基因决定；它只是说在这个群体里，差异主要来自基因
在对遗传性研究的批评中，常提到“控制变量”的问题：
实验往往是在相似的社会背景（如中产阶级家庭）下进行，这样一来环境变异被控制住了，实验所看到的大多数差异自然就被归因于基因
批评的潜台词是：现实中的社会环境并不均等，社会阶层差异、教育机会不均、营养差距等，都会显著影响个体表现，而不仅仅是基因因此，遗传性在这些实验中被高估了
但从另一个角度看，这个现象也反映出一个核心逻辑：
如果社会环境真正实现了公平、均等，那么人们的表现差异将更主要地来源于“先天”——也就是基因
举例来说，在一个所有人都能获得良好营养的社会，身高的差异将几乎完全来自遗传；在一个教育资源均衡分布的社会，智力测验成绩的差异也将更多由遗传因素决定
因此，在一个越“公平公正”的社会中，遗传性反而会变得更高
这不是因为基因变得更重要了，而是因为环境因素变得更一致、影响力更小了
这对原先的批判的进行了重新诠释——真正的平等让先天差异更加突出
Heritability ≠ fatalism (still subject to environmental influences)
The discovery of genetic mutations behind conditions like dyslexia has not led teachers to abandon such conditions as incurable—quite the reverse; it has encouraged them to single out dyslexic children for special teaching. Indeed, the most famous pioneer of intelligence testing, the Frenchman Alfred Binet, argued fervently that its purpose was not to reward gifted children but to give special attention to less gifted ones.
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