"But back to the question: What does heritability mean? Almost everyone who has ever thought about heritability has reached a commonsense intuition about it: One way or another, heritability has to be some kind of index of how genetic a trait is. That intuition explains why so many thousands of heritability coefficients have been calculated over the years. . . .
"Unfortunately, that fundamental intuition is wrong. Heritability isn't an index of how genetic a trait is. A great deal of time has been wasted in the effort of measuring the heritability of traits in the false expectation that somehow the genetic nature of psychological phenomena would be revealed. There are many reasons for making this strong statement, but the most important of them harkens back to the description of heritability as an effect size. An effect size of the R2 family is a standardized estimate of the proportion of the variance in one variable that is reduced when another variable is held constant statistically. In this case it is an estimate of how much the variance of a trait would be reduced if everyone were genetically identical. With a moment's thought you can see that the answer to the question of how much variance would be reduced if everyone was genetically identical depends crucially on how genetically different everyone was in the first place."
Johnson, Wendy; Turkheimer, Eric; Gottesman, Irving I.; Bouchard Jr., Thomas (2009). Beyond Heritability: Twin Studies in Behavioral Research. Current Directions in Psychological Science, 18, 4, 217-220
"Moreover, even highly heritable traits can be strongly manipulated by the environment, so heritability has little if anything to do with controllability. For example, height is on the order of 90% heritable, yet North and South Koreans, who come from the same genetic background, presently differ in average height by a full 6 inches (Pak, 2004; Schwekendiek, 2008)."
Deary, I. J., Penke, L., & Johnson, W. (2010). The neuroscience of human intelligence differences. Nature Reviews Neuroscience, 11(3), 201-211.
"At this point, it seems unlikely that single genetic loci have major effects on normal-range intelligence. For example, a modestly sized genome-wide study of the general intelligence factor derived from ten separate test scores in the cAnTAB cognitive test battery did not find any important genome-wide single nucleotide polymorphisms or copy number variants, and did not replicate genetic variants that had previously been associated with cognitive ability[note 48]."
Johnson, W. (2010). Understanding the Genetics of Intelligence: Can Height Help? Can Corn Oil?. Current Directions in Psychological Science, 19(3), 177-182
"Together, however, the developmental natures of GCA [general cognitive ability] and height, the likely influences of gene-environment correlations and interactions on their developmental processes, and the potential for genetic background and environmental circumstances to release previously unexpressed genetic variation suggest that very different combinations of genes may produce identical IQs or heights or levels of any other psychological trait. And the same genes may produce very different IQs and heights against different genetic backgrounds and in different environmental circumstances. This would be especially the case if height and GCA and other psychological traits are only single facets of multifaceted traits actually under more systematic genetic regulation, such as overall body size and balance between processing capacity and stimulus reactivity. Genetic influences on individual differences in psychological characteristics are real and important but are unlikely to be straightforward and deterministic."
Johnson, W., Penke, L., & Spinath, F. M. (2011). Understanding Heritability: What it is and What it is Not. European Journal of Personality, 25(4), 287-294. DOI: 10.1002/per.835
"Our target article was intended to provide background knowledge to psychologists and other social scientists on the subject of heritability. This statistic, in many ways so basic, is both extremely powerful in revealing the presence of genetic influence and very weak in providing much information beyond this. Many forms of measurement error, statistical artefact, violation of underlying assumptions, gene–environment interplay, epigenetic mechanisms and no doubt processes we have not yet even identified can contribute to the magnitudes of heritability estimates. If psychologists and other social scientists want to understand genetic involvement in behavioural traits, we believe that it is going to be necessary to distinguish among these possibilities to at least some degree. Heritability estimates alone are not going to help us do this."
Turkheimer, E. (2011). Genetics and human agency (Commentary on Dar-Nimrod & Heine, 2011). Psychological Bulletin, 137, 825-828. DOI: 10.1037/a0024306
"That heritability depends on the population in which it is measured is one of the most frequently repeated caveats in the social sciences, but it is nevertheless often forgotten in the breach. (For example, it is nearly meaningless for Dar-Nimrod and Heine to note that 'heritability [of intelligence is] typically estimated to range from .50 to .85' [p. 805]. The heritability of intelligence isn’t anything, and even placing it in a range is misleading. Making a numerical point estimate of the heritability of intelligence is akin to saying, 'Social psychologists usually estimate the F ratio for the fundamental attribution error to be between 2.0 and 4.0.') The observation that genotypic variation accounts for 90% of the variation in height in the modern world depends on the variability of genotype and environment relevant to height. Among cloned animals with widely varying diets, body size is perfectly environmental with heritability of 0; in genetically variable animals raised in identical environments heritability is 1.0. This is no mere statistical fine point: it means that the entire project of assessing how essentially genetic traits are in terms of measured heritability coefficients is a fool’s errand."
Chabris, C. F., Hebert, B. M., Benjamin, D. J., Beauchamp, J., Cesarini, D., van der Loos, M., ... & Laibson, D. (2012). Most reported genetic associations with general intelligence are probably false positives. Psychological science, 23(11), 1314-1323. DOI: 10.1177/0956797611435528
"At the time most of the results we attempted to replicate were obtained, candidate-gene studies of complex traits were commonplace in medical genetics research. Such studies are now rarely published in leading journals. Our results add IQ to the list of phenotypes that must be approached with great caution when considering published molecular genetic associations. In our view, excitement over the value of behavioral and molecular genetic studies in the social sciences should be tempered—as it has been in the medical sciences—by an appreciation that, for complex phenotypes, individual common genetic variants of the sort assayed by SNP microarrays are likely to have very small effects. Associations of candidate genes with psychological traits and other traits studied in the social sciences should be viewed as tentative until they have been replicated in multiple large samples. Doing otherwise may hamper scientific progress by proliferating potentially false results, which may then influence the research agendas of scientists who do not appreciate that the associations they take as a starting point for their efforts may not be real. And the dissemination of false results to the public risks creating an incorrect perception about the state of knowledge in the field, especially the existence of genes described as being 'for' traits on the basis of unintentionally inflated estimates of effect size and statistical significance."
I really am not impressed with arguments that you can’t measure the inheritance of intelligence because the individuals don’t share a common environment. If this was true you would not be able to do any genetic studies on any trait.
The more important question is how much shared environment contributes to the variation in intelligence between individuals and the answer to that is basically zero.
http://people.virginia.edu/~ent3c/papers2/Articles%20for%20O...
"But back to the question: What does heritability mean? Almost everyone who has ever thought about heritability has reached a commonsense intuition about it: One way or another, heritability has to be some kind of index of how genetic a trait is. That intuition explains why so many thousands of heritability coefficients have been calculated over the years. . . .
"Unfortunately, that fundamental intuition is wrong. Heritability isn't an index of how genetic a trait is. A great deal of time has been wasted in the effort of measuring the heritability of traits in the false expectation that somehow the genetic nature of psychological phenomena would be revealed. There are many reasons for making this strong statement, but the most important of them harkens back to the description of heritability as an effect size. An effect size of the R2 family is a standardized estimate of the proportion of the variance in one variable that is reduced when another variable is held constant statistically. In this case it is an estimate of how much the variance of a trait would be reduced if everyone were genetically identical. With a moment's thought you can see that the answer to the question of how much variance would be reduced if everyone was genetically identical depends crucially on how genetically different everyone was in the first place."
Johnson, Wendy; Turkheimer, Eric; Gottesman, Irving I.; Bouchard Jr., Thomas (2009). Beyond Heritability: Twin Studies in Behavioral Research. Current Directions in Psychological Science, 18, 4, 217-220
http://people.virginia.edu/~ent3c/papers2/Articles%20for%20O...
"Moreover, even highly heritable traits can be strongly manipulated by the environment, so heritability has little if anything to do with controllability. For example, height is on the order of 90% heritable, yet North and South Koreans, who come from the same genetic background, presently differ in average height by a full 6 inches (Pak, 2004; Schwekendiek, 2008)."
Deary, I. J., Penke, L., & Johnson, W. (2010). The neuroscience of human intelligence differences. Nature Reviews Neuroscience, 11(3), 201-211.
http://www.larspenke.eu/pdfs/Deary_Penke_Johnson_2010_-_Neur...
"At this point, it seems unlikely that single genetic loci have major effects on normal-range intelligence. For example, a modestly sized genome-wide study of the general intelligence factor derived from ten separate test scores in the cAnTAB cognitive test battery did not find any important genome-wide single nucleotide polymorphisms or copy number variants, and did not replicate genetic variants that had previously been associated with cognitive ability[note 48]."
Johnson, W. (2010). Understanding the Genetics of Intelligence: Can Height Help? Can Corn Oil?. Current Directions in Psychological Science, 19(3), 177-182
http://apsychoserver.psych.arizona.edu/JJBAReprints/PSYC621/...
"Together, however, the developmental natures of GCA [general cognitive ability] and height, the likely influences of gene-environment correlations and interactions on their developmental processes, and the potential for genetic background and environmental circumstances to release previously unexpressed genetic variation suggest that very different combinations of genes may produce identical IQs or heights or levels of any other psychological trait. And the same genes may produce very different IQs and heights against different genetic backgrounds and in different environmental circumstances. This would be especially the case if height and GCA and other psychological traits are only single facets of multifaceted traits actually under more systematic genetic regulation, such as overall body size and balance between processing capacity and stimulus reactivity. Genetic influences on individual differences in psychological characteristics are real and important but are unlikely to be straightforward and deterministic."
Johnson, W., Penke, L., & Spinath, F. M. (2011). Understanding Heritability: What it is and What it is Not. European Journal of Personality, 25(4), 287-294. DOI: 10.1002/per.835
http://www.larspenke.eu/pdfs/Johnson_Penke_Spinath_2011_-_He...
"Our target article was intended to provide background knowledge to psychologists and other social scientists on the subject of heritability. This statistic, in many ways so basic, is both extremely powerful in revealing the presence of genetic influence and very weak in providing much information beyond this. Many forms of measurement error, statistical artefact, violation of underlying assumptions, gene–environment interplay, epigenetic mechanisms and no doubt processes we have not yet even identified can contribute to the magnitudes of heritability estimates. If psychologists and other social scientists want to understand genetic involvement in behavioural traits, we believe that it is going to be necessary to distinguish among these possibilities to at least some degree. Heritability estimates alone are not going to help us do this."
Turkheimer, E. (2011). Genetics and human agency (Commentary on Dar-Nimrod & Heine, 2011). Psychological Bulletin, 137, 825-828. DOI: 10.1037/a0024306
http://people.virginia.edu/~ent3c/papers2/Articles%20for%20O...
"That heritability depends on the population in which it is measured is one of the most frequently repeated caveats in the social sciences, but it is nevertheless often forgotten in the breach. (For example, it is nearly meaningless for Dar-Nimrod and Heine to note that 'heritability [of intelligence is] typically estimated to range from .50 to .85' [p. 805]. The heritability of intelligence isn’t anything, and even placing it in a range is misleading. Making a numerical point estimate of the heritability of intelligence is akin to saying, 'Social psychologists usually estimate the F ratio for the fundamental attribution error to be between 2.0 and 4.0.') The observation that genotypic variation accounts for 90% of the variation in height in the modern world depends on the variability of genotype and environment relevant to height. Among cloned animals with widely varying diets, body size is perfectly environmental with heritability of 0; in genetically variable animals raised in identical environments heritability is 1.0. This is no mere statistical fine point: it means that the entire project of assessing how essentially genetic traits are in terms of measured heritability coefficients is a fool’s errand."
Chabris, C. F., Hebert, B. M., Benjamin, D. J., Beauchamp, J., Cesarini, D., van der Loos, M., ... & Laibson, D. (2012). Most reported genetic associations with general intelligence are probably false positives. Psychological science, 23(11), 1314-1323. DOI: 10.1177/0956797611435528
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3498585/
http://dash.harvard.edu/bitstream/handle/1/9938142/Most_Repo...
"At the time most of the results we attempted to replicate were obtained, candidate-gene studies of complex traits were commonplace in medical genetics research. Such studies are now rarely published in leading journals. Our results add IQ to the list of phenotypes that must be approached with great caution when considering published molecular genetic associations. In our view, excitement over the value of behavioral and molecular genetic studies in the social sciences should be tempered—as it has been in the medical sciences—by an appreciation that, for complex phenotypes, individual common genetic variants of the sort assayed by SNP microarrays are likely to have very small effects. Associations of candidate genes with psychological traits and other traits studied in the social sciences should be viewed as tentative until they have been replicated in multiple large samples. Doing otherwise may hamper scientific progress by proliferating potentially false results, which may then influence the research agendas of scientists who do not appreciate that the associations they take as a starting point for their efforts may not be real. And the dissemination of false results to the public risks creating an incorrect perception about the state of knowledge in the field, especially the existence of genes described as being 'for' traits on the basis of unintentionally inflated estimates of effect size and statistical significance."