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Twin studies are a good way to get another angle on this. Here, unfortunately that doesn't get around the confounding factor that only parents deemed suitable

Twin studies are a good way to get another angle on this. Here, unfortunately that doesn't get around the confounding factor that only parents deemed suitable candidates by adoption agencies are able to adopt and raise children, so they are not an accurate cross section of all possible parents. I don't dispute that there may be a threshhold at which some innate traits take over (or fail to do so) from the benefits of a nurturing environment (which may have diminishing returns past a certain point).

mlyle 4y ago
Terrible neglect results in low IQ. No one is arguing that. Obviously, whacking the kid on the head with a hammer, giving her terribly insufficient food, or never exposing her to language will depress IQ. These are all things that parents sometimes do. Not only do we have evidence that the heritability of intelligence is high, from twin studies, but also: adopted siblings' intelligence is only slightly correlated-- barely better than strangers. Biological sibling intelligence raised in the same house are very closely correlated. This seems to imply once parenting is "good enough", these types of environmental effects don't affect IQ in adulthood to a significant degree. Perhaps the most interesting finding: parenting does seem to affect IQ scores in early childhood more than genetics, but by adulthood the strengths of these relationships have completely reversed.
- dwringer 4y ago
  I appreciate your well reasoned and thoughtful reply and I would like to add that I find your assessment completely plausible and I myself tend to agree with it. Although, I still question the validity of using IQ as a universal objective measurement of intelligence. At best I can see there being a fairly wide pareto front of optimal intelligence for most situations. But who can ultimately judge? Into adulthood I find it very difficult to say whose intelligence is "better" in anything close to an objective way. While the results of genetic influence may become evident in some kinds of tests, it is just as easy to conceive tests at which the same candidate would fail yet someone else with a lower IQ might do very well. The possibilities are endless and I really don't think outside of cases of abuse or neglect creating specific developmental deficiencies as you mentioned that we can meaningfully quantify intelligence generally. If we could better do so I think the influence of parenting might still become more evident than what we see in IQ tests. > No one is arguing that. Well, my initial comment was simply an attempt to make the point that parenting can have an effect whether or not genetics do and the reply I got was "look it up".
  - mlyle 4y ago
    > Into adulthood I find it very difficult to say whose intelligence is "better" in anything close to an objective way. Intelligence is difficult to quantify and define. BUT: peoples abilities on a wide variety of tasks are positively correlated. This strongly implies there is some kind of underlying factor that makes you better at "all things", and this is what IQ tests attempt to measure. e.g. see: https://en.wikipedia.org/wiki/G_factor_(psychometrics)#/media/File:Chabris2007a.svg https://en.wikipedia.org/wiki/G_factor_(psychometrics)#/medi... Of course, the correlation coefficients are not 1; so of course someone can have a lower IQ than you but far outperform you in one of these tasks. The results from IQ tests seem to be very strong predictors of many outcome measures later in life-- indeed, more than parenting seems to predict these things. edit: from the wikipedia article on g factor: " Research indicates that tests of g are the best single predictors of job performance, with an average validity coefficient of .55 across several meta-analyses of studies based on supervisor ratings and job samples. The average meta-analytic validity coefficient for performance in job training is .63.[76] The validity of g in the highest complexity jobs (professional, scientific, and upper management jobs) has been found to be greater than in the lowest complexity jobs, but g has predictive validity even for the simplest jobs. Research also shows that specific aptitude tests tailored for each job provide little or no increase in predictive validity over tests of general intelligence."

mlyle 4y ago

Terrible neglect results in low IQ. No one is arguing that. Obviously, whacking the kid on the head with a hammer, giving her terribly insufficient food, or never exposing her to language will depress IQ. These are all things that parents sometimes do. Not only do we have evidence that the heritability of intelligence is high, from twin studies, but also: adopted siblings' intelligence is only slightly correlated-- barely better than strangers. Biological sibling intelligence raised in the same house are very closely correlated. This seems to imply once parenting is "good enough", these types of environmental effects don't affect IQ in adulthood to a significant degree. Perhaps the most interesting finding: parenting does seem to affect IQ scores in early childhood more than genetics, but by adulthood the strengths of these relationships have completely reversed.

dwringer 4y ago
I appreciate your well reasoned and thoughtful reply and I would like to add that I find your assessment completely plausible and I myself tend to agree with it. Although, I still question the validity of using IQ as a universal objective measurement of intelligence. At best I can see there being a fairly wide pareto front of optimal intelligence for most situations. But who can ultimately judge? Into adulthood I find it very difficult to say whose intelligence is "better" in anything close to an objective way. While the results of genetic influence may become evident in some kinds of tests, it is just as easy to conceive tests at which the same candidate would fail yet someone else with a lower IQ might do very well. The possibilities are endless and I really don't think outside of cases of abuse or neglect creating specific developmental deficiencies as you mentioned that we can meaningfully quantify intelligence generally. If we could better do so I think the influence of parenting might still become more evident than what we see in IQ tests. > No one is arguing that. Well, my initial comment was simply an attempt to make the point that parenting can have an effect whether or not genetics do and the reply I got was "look it up".
- mlyle 4y ago
  > Into adulthood I find it very difficult to say whose intelligence is "better" in anything close to an objective way. Intelligence is difficult to quantify and define. BUT: peoples abilities on a wide variety of tasks are positively correlated. This strongly implies there is some kind of underlying factor that makes you better at "all things", and this is what IQ tests attempt to measure. e.g. see: https://en.wikipedia.org/wiki/G_factor_(psychometrics)#/media/File:Chabris2007a.svg https://en.wikipedia.org/wiki/G_factor_(psychometrics)#/medi... Of course, the correlation coefficients are not 1; so of course someone can have a lower IQ than you but far outperform you in one of these tasks. The results from IQ tests seem to be very strong predictors of many outcome measures later in life-- indeed, more than parenting seems to predict these things. edit: from the wikipedia article on g factor: " Research indicates that tests of g are the best single predictors of job performance, with an average validity coefficient of .55 across several meta-analyses of studies based on supervisor ratings and job samples. The average meta-analytic validity coefficient for performance in job training is .63.[76] The validity of g in the highest complexity jobs (professional, scientific, and upper management jobs) has been found to be greater than in the lowest complexity jobs, but g has predictive validity even for the simplest jobs. Research also shows that specific aptitude tests tailored for each job provide little or no increase in predictive validity over tests of general intelligence."