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. Author manuscript; available in PMC: 2020 Aug 20.
Published in final edited form as: Intelligence. 2019 May 7;75:48–58. doi: 10.1016/j.intell.2019.01.011

Table 2:

Within-family associations between brain/head size and IQ.

Dataset Families Measure β ± SE ρ p-value
HCP
 All sibs 381 Brain volume 0.142 ± 0.040 0.18 4 × 10−4
 MZ 138 Brain volume 0.175 ± 0.168 0.22 0.30
 DZ 79 Brain volume 0.055 ± 0.109 0.07 0.61
 Non-twins 161 Brain volume 0.162 ± 0.064 0.21 0.01
MCTFR
 All sibs 1,349 Head circumference 0.172 ± 0.031 0.19 3 × 10−8
 MZ 11 yr 642 Head circumference 0.150 ± 0.049 0.17 0.002
 DZ 11 yr 363 Head circumference 0.219 ± 0.053 0.24 5 × 10−5
 MZ 17 yr 223 Head circumference −0.050 ± 0.086 −0.06 0.56
 DZ 17 yr 121 Head circumference 0.209 ± 0.104 0.23 0.05

β is the estimated partial regression coefficient of brain/head size in a model predicting IQ with family fixed effects. Both size and IQ were standardized. “All sibs” means all families in the dataset, regardless of type (e.g., zygosity). Within each dataset, the rows beneath “All sibs” correspond to subsamples of the total sample. ρ, the partial regression coefficient (β) divided by the square root of the IQ test’s internal-consistency reliability.