While it is almost certain that intelligence ahs a neurological basis, the extent to which intelligence is determined by neurological and chemical changes is not fully understood. A recent study suggests that changing hormone levels prior to and during puberty have a large effect on intelligence; high salivary testosterone levels in ten-year-old boys showed a positive correlation with intelligence, but a negative correlation in twelve-year-olds (Shangguan & Shi 2009). This shows that intelligence can be quite fluid, and can even be affected quite differently by the same mechanisms. The exact nature of the changing influence testosterone seems to provide or signal is not known, but the correlation demands further study. These three different research studies display very different theories of intelligence. In the first study, accepted methods of cognitive relationships with the environment are valid, if perhaps somewhat inaccurate at the extremes of the intelligence spectrum. The second study points out the unnecessary effects of such inaccuracies when more accurate measures of cognitive performance are available; this could be considered something of a "memory theory" of intelligence (CJ 2009). The final study examines the purely physiological basis of intelligence, purposefully setting aside environmental influences and focusing on chemical factors. Taken as a while, these studies show several of the different theories and perspectives that can be brought to bear on the issue...

This is not a problem from a purely scientific point-of-view; science is progressed through such disagreements and resultant theoretical shifts and refinements. Given the practical and influential uses to which intelligence tests are put, however, there is a large practical pressure to define intelligence as accurately as possible as quickly as possible. Despite -- in fact because of -- their disagreement, these studies promise to further that end.
References

CJ. (2009). "Working memory predicts learning outcomes." Psychologist 22910), pp. 827.

Shangguan, F. & Shi, J. (2009). "Puberty timing and fluid intelligence: A study of correlations between testosterone and intelligence in 8- to 12-year-old Chinese boys." Psychoneuroendocrinology 34(7), pp. 983-8.

Whitaker, S. & Wood, J. (2007). "The Distribution of Scaled Scores and Possible Floor Effects on the WISC-III and WAIS-III." Journal of applied research in intellectual disabilities, 21(2), pp.…