of a test taker and those of their biological parents. However, since high heritability is simply a correlation between child and parents, it does not describe the causes of heritability which in humans can be either genetic or environmental. Therefore, a high heritability measure does not imply that a trait is genetic or unchangeable. In addition, environmental factors that affect all group members equally will not be measured by heritability, and the heritability of a trait may also change over time in response to changes in the distribution of genetic and environmental factors. High heritability does not imply that all of the heritability is genetically determined; rather, it can also be due to environmental differences that affect only a certain genetically defined group (indirect heritability). The figure to the right demonstrates how heritability works. In each of the two gardens the difference between tall and short cornstalks is 100% heritable, as cornstalks that are genetically disposed for growing tall will become taller than those without this disposition. But the difference in height between the cornstalks to the left and those on the right is 100% environmental, as it is due to different nutrients being supplied to the two gardens. Hence, the causes of differences within a group and between groups may not be the same, even when looking at traits that are highly heritable. Spearman's hypothesis Spearman's hypothesis states that the magnitude of the black–white difference in tests of cognitive ability depends entirely or mainly on the extent to which a test measures general mental ability, or g. The hypothesis was first formalized by Arthur Jensen, who devised the statistical "method of correlated vectors" to test it. If Spearman's hypothesis holds true, then the cognitive tasks that have the highest g-load are the tasks in which the gap between black and white test takers are greatest. Jensen and Rushton take this to show that the cause of g and the cause of the gap are the same—in their view, genetic differences. acknowledges that Jensen and Rushton showed a modest correlation between g-loading, heritability, and the test score gap, but does not agree that this demonstrates a genetic origin of the gap. Mackintosh argues that it is exactly those tests that Rushton and Jensen consider to have the highest g-loading and heritability, such as the Wechsler test, that have seen the greatest increases in black performance due to the Flynn effect.