Table 2.

Proportional reduction of sum of squared residuals associated with addition of age effects for analyses of relative brain-region size

Brain region	Age model	Age effects in chimpanzees (n = 99)	Age effects in humans (n = 87)
Relative frontal lobe gray matter	Linear	0.001 (0.811)	0.109 (0.002*)
	Quadratic	0.004 (0.844)	0.135 (0.002*)
	Cubic	0.012 (0.782)	0.141 (0.006*)
Relative hippocampus	Linear	0.007 (0.429)	0.019 (0.206)
	Quadratic	0.012 (0.585)	0.059 (0.079)
	Cubic†	0.023 (0.538)
	Males	—	0.181 (0.048*)
	Females	—	0.176 (0.050*)
Relative frontal lobe white matter	Linear‡		0.053 (0.038*)
	1.5 T	0.000 (0.913)	—
	3 T	0.097 (0.073)	—
	PM 3 T	0.237 (0.007*)	—
	Quadratic‡		0.065 (0.061)
	1.5 T	0.016 (0.788)	—
	3 T	0.109 (0.167)	—
	PM 3 T	0.365 (0.003*)	—
	Cubic	0.132 (0.004*)	0.079 (0.079)

Proportional reduction of sum of squared residuals associated with addition of age effects (as explained in Table 1) for analyses of relative brain region size. Before the addition of age effects, relative brain-region size is dependent on sex in humans, and is dependent on sex and MRI scan type in chimpanzees. All of the models reported here do not include interaction terms. Abbreviations follow Table 1. Boldface indicates best-fit model following procedure described in Materials and Methods; an asterisk indicates the age effect is significant at α = 0.05; em-dash indicates no data.

^†Significant interaction between sex and age; regression model run separately within each sex. PM, postmortem.

^‡Significant interaction between MRI scan type and age variables; regression model run separately within each MRI scan type.