[btilly]

BTW evolution theory suggests a priori that the Continuity Hypothesis is more likely to be correct.

Intelligence is selected for in humans. Therefore any individual gene that significantly boosts intelligence should be expected to have already spread through the population. As a result we should not expect to find any rare genes that make people super-smart. So super-smart people get there with a combination of different genes, each of which contributes very little.

The bottom end of the scale is the opposite story. Evolution says that individual genes that hurt intelligence should be selected against, and are therefore expected to be rare. (Mutation says that they should not be non-existent, but they should be rare.) Therefore there is no surprise in finding rare individual genes that significantly hurt intelligence.

This pattern is not unique to intelligence. It is predicted for any trait that has actively been selected for by evolution over a long period. The top of the range should look continuous. The bottom of the range tends to be dominated by deleterious point mutations.

[idanoeman]

Intelligence is selected for in humans. Therefore any individual gene that significantly boosts intelligence should be expected to have already spread through the population, or has a trade-off that reduces reproductive fitness.

The same can be said with drugs -- any chemical that your body could have reasonably produced on its own should be expected to reduce your reproductive fitness. So brain drugs should nearly all have side-effects.