Intelligence: Nature vs Nurture

Rebuttals: 1. "More recent and better conducted studies have found far less statistical 'heritability, ' (Adoption results for self-reported personality: evidence for nonadditive genetic effects? Plomin R. Et al)" My opponent seems to have made a blunder as this study’s findings are not relevant to our debate as it based on children aged 9-16, While we are, As I laid out in round one, Only arguing about “human adult intelligence”. Especially since the lead researcher of the study that you have cited, Robert Plomin, Pointed out himself that “the heritability of intelligence increases throughout development”. What is even more interesting is that Plomin later established that the heritability of intelligence in adulthood is around 60% (Plomin, DeFries, Knopik, & Neiderhiser, 2016a) which leaves a maximum influence of 40% for nurture. 2. “So we arrive at last. Your studies have not held to peer review or criticisms. ” 2. 1 Peer review: If my opponent’s claim that the findings of MISTRA and the findings of Horn’s Texas Adoption Project are not peer-reviewed is true, This would question their findings. However, My opponent is mistaken. The findings of MISTRA were published in the peer-reviewed academic journal “Science”, Which is one of the world’s top academic journals, And the findings of the Texas Adoption Project were published in “Behavior Genetics” and “Child Development”, Both well-respected peer-reviewed academic journals. 2. 2 Criticism: Are the findings of MISTRA and the Texas Adoption Project, That nature plays a bigger role than nurture in creating intelligence differences among individuals, Generally accepted, Or generally rejected in the scientific community? To investigate this, I will present three of the most influential surveys about expert opinion on intelligence: Dataset 1: Neisser et al. (1996) In 1995 the American Psychological Association (APA), The largest and most authoritative professional psychological society (Dreary, 2001), Issued a report titled “Intelligence: Knowns and Unknowns” in response to the “serious misunderstandings” in the public about intelligence. They appointed a task force to establish through consensus among psychologists what was known and what was not known about human intelligence. The report had the unanimous support of the entire task force and established several “knowns” about intelligence. 1. The report cited both MISTRA and the Texas Adoption Study, Showing that both are accepted in mainstream psychology and the leading experts in intelligence research, Contrary to my opponent’s claim. 2. One of the “knowns” is that the heritability of intelligence in adulthood is around 75%, Which leaves a maximum influence of 25% for nurture. Dataset 2: Gottfredson (1994) Gottfredson (1994) aimed to outline some conclusions that are considered mainstream among researchers on intelligence. A conclusion of Gottfredson (1994) is that there is a general agreement among researchers that nature plays a bigger role than nurture in creating IQ differences among individuals. Dataset 3: Snyderman and Rothman (1987) Stanley Rothman and Mark Snyderman surveyed 1020 experts (psychologists, Sociologists and educationalists) on their views on intelligence and aptitude testing. The experts were presented with a list of five sources of evidence for the heritability of intelligence and were asked to check all sources that they believed provided reasonable support for the heritability of intelligence. 1. 84. 4% of the respondents checked that MISTRA provided reasonable support for the heritability of intelligence, Indicating that MISTRA is very well accepted among experts. 2. Respondents were also asked for their estimates of the heritability of intelligence (among white people and separately among black people), The mean answer for both white and black heritability was around 60%, Which leaves a maximum influence of 40% for nurture. 3. “Scientific studies though, Need to have results repeated over time in order to be deemed factual. ” My opponent is indeed correct in claiming that if studies are not successfully replicated, They lose credibility. Let us thus investigate whether they have been replicated: Dataset 4: Plomin, DeFries, Knopik, & Neiderhiser, 2016a In the scientific paper fittingly titled “Top 10 Replicated Findings From Behavioral Genetics”, It was described that the heritability of intelligence being between 50% and 80% in adulthood is one of the findings that has been replicated robustly in behavioural genetics, Suggesting that my opponent’s assertions that further replication is required to substantiate these findings is unwarranted. Dataset 5: Haworth et al. , (2009) In a huge sample of 11 000 pairs of twins (a larger sample than in all previous studies on the heritability of intelligence combined) from four different countries (leading to an even bigger difference in environments) it was found that the heritability of intelligence increases to 66% in young adulthood, Which leaves a maximum influence of 34% for nurture and shows that the findings have indeed been recently replicated on a large scale, With a sample that is representative of several cultures. 4. They can't point to specific genes that would make people do any of what they assume it does. It operates on a hunch, For now. My opponent makes it seem as if it was expected that one could point to specific genes, This is however not the case as Deary (2001) explains that intelligence is influenced by an unquantifiable number of genes each of which has a small effect and that thus, It is unreasonable to expect researchers to be able to specific genes that account for much of the variability in intelligence between individuals. This is also supported by Plomin, DeFries, Knopik, & Neiderhiser (2016a) which established that one of the findings that has been robustly replicated in behavioural genetics is that heritability is caused by genes of small effects. 5. What my opponent is doing is giving everyone the same nurture by saying "on average" and then saying nature is more impactful. I can simply say that 'on average' everyone would have identical natures as well and this debate is irrelevant. I do not see how my opponent came to this conclusion as if the “on average” meant that everyone has the same nature then nature would have no detectable influence and if the “on average” implied that everyone had the same nature than surely it would also imply that everyone had the same nurture. I assume this is not what my opponent is intending to say and thus suggest that he dropped this, On average simply means that a sample that is somewhat representative of the general population should be used, Instead of a sample that consists purely of hyper-intelligent or mentally-challenged people. Conclusion: As studies on identical twins have found that 70% of the variance in IQ scores is accounted for by genes (see Bouchard et al. , 1990; and Dataset 4), Adoption studies have found that the heritability of IQ is above 60% (see Datasets 4 and 5) and surveys have shown that there is a general acceptance of the evidence of the heritability of intelligence and a consensus between experts in the field of intelligence research that nature has a larger influence than nature on human adult intelligence (see Datasets 1, 2 and 3) exists, While my opponent was only able to present the criticisms of one cherry-picked researcher, Jay Joseph, Whose publications on MISTRA and the Texas Adoption Project have not all been peer-reviewed (in contrast to the studies that I have presented; see rebuttal 2. 1) and are in conflict with mainstream science (as his assertions are contrary to the consensus), It can be concluded that nature, On average, Has a more significant impact on human adult intelligence than nurture.