A classic study [15] of the IQ scores of 34 identical twins who were raised apart used the intelligence scores in Table 8.18. Draw a scatter diagram and calculate the R² for a regression of the intelligence score of the second-born on the intelligence score of the first-born. (This R² was used to measure the influence of genes on intelligence, with 1 − R² measuring the influence of environment.) If 100 were added to all the first-born IQ scores or all the second-born IQ scores, how do you think R2 would be affected?

Check your expectations by adding 100 to all the IQs and recalculating R².

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Table 8.18: Exercise 8.47 First Second First Second First Second First Second First Second 22 12 30 26 30 34 6 10 41 41 36 34 29 35 27 24 23 21 19 9 13 10 26 20 32 18 38 27 40 38 30 25 28 22 27 28 33 26 12 9 32 28 21 27 22 23 16 28 26 17 13 4 15 9 27 25 22 13 22 29 30 20 24 32 33 24 33 4 2