Relationship between IQ and Brain Size

Summary:
	Monozygotic twins share numerous physical, psychological, and pathological traits.  Recent advances in in vivo brain image acquisition and analysis have made it possible to determine quantitatively whether: 1) twins share neuroanatomical traits; and 2) neuroanatomical measures correlate with brain size.
	Using magnetic resonance imaging and computer-based image analysis techniques, measurements of the volume of the forebrain, the surface area of the cerebral cortex and the mid-sagittal area of the corpus callosum were obtained in 10 pairs of monozygotic twins.  Head circumference, body weight, and Full-Scale IQ were also measured.  Analyses of variance were carried out using genotype, birth order, and sex, as between-subject factors.  Pearson correlation coefficients were computed to assess the interrelationships between brain measures, head circumference, and IQ.
	Effects of genotype (but not of birth order) were found for total forebrain volume, total cortical surface area, and callosal area.  Consistent with previous twin studies, highly significant effects of genotype but not birth order were also found for head circumference, body weight, and Full-Scale IQ.  The significant effect of genotype on all measures was not attributable to sex differences across unrelated twin pairs.  Significant correlations were observed between forebrain volume, cortical surface area, and callosal area as well as between each brain measure and head circumference.  No correlation between IQ and any other measure was found.  
	Monozygotic twins share similarities in forebrain volume, cortical surface area, and callosal area.  Brain measures are highly correlated with one another and with head circumference, but none is correlated with IQ.

Authorization: Contact Authors 

Reference:
Tramo MJ, Loftus WC, Green RL, Stukel TA, Weaver JB, Gazzaniga MS.  Brain Size, Head Size, and IQ in Monozygotic Twins.  Neurology  1998; 50:1246-1252.

Description:  This datafile contains 20 observations (10 pairs of twins) on 9 variables.  This data set can be used to demonstrate simple linear regression and correlation.


Variable Names in order from left to right: 
	CCMIDSA: Corpus Collasum Surface Area (cm2)
	FIQ: Full-Scale IQ
	HC: Head Circumference (cm)
	ORDER: Birth Order
	PAIR: Pair ID (Genotype)
	SEX: Sex (1=Male 2=Female)
	TOTSA: Total Surface Area (cm2)
	TOTVOL: Total Brain Volume (cm3)
	WEIGHT: Body Weight (kg)

6.08	96	54.7	1	1	2	1913.88	1005	57.607
5.73	89	54.2	2	1	2	1684.89	963	58.968
6.22	87	53	1	2	2	1902.36	1035	64.184
5.8	87	52.9	2	2	2	1860.24	1027	58.514
7.99	101	57.8	1	3	2	2264.25	1281	63.958
8.42	103	56.9	2	3	2	2216.4	1272	61.69
7.44	103	56.6	1	4	2	1866.99	1051	133.358
6.84	96	55.3	2	4	2	1850.64	1079	107.503
6.48	127	53.1	1	5	2	1743.04	1034	62.143
6.43	126	54.8	2	5	2	1709.3	1070	83.009
7.99	101	57.2	2	6	1	1689.6	1173	61.236
8.76	96	57.2	1	6	1	1806.31	1079	61.236
6.32	93	57.2	2	7	1	2136.37	1067	83.916
6.32	88	57.2	1	7	1	2018.92	1104	79.38
7.6	94	55.8	2	8	1	1966.81	1347	97.524
7.62	85	57.2	1	8	1	2154.67	1439	99.792
6.03	97	57.2	1	9	1	1767.56	1029	81.648
6.59	114	56.5	2	9	1	1827.92	1100	88.452
7.52	113	59.2	2	10	1	1773.83	1204	79.38
7.67	124	58.5	1	10	1	1971.63	1160	72.576


Therese Stukel
Dartmouth Hitchcock Medical Center
One Medical Center Dr.
Lebanon, NH 03756
e-mail: stukel@dartmouth.edu