... and the ability to be trained into learning arbitrary skills is IQ. Higher IQ means its easier to train yourself into being able to use a new system of thinking.

Like, being a soldier doesn't require any sort of exceptional intelligence either. It doesn't even require average intelligence. What it requires is an IQ of 85 - below that point, and the US army cannot effectively train you to become a soldier. Despite having every incentive in the world, the US army still rejects about 15% of the population.

15% of the population doesn't apply to join the Army. And those that do self select. Using the Army rejection rates for determining the IQ level of the population isn't a good method.

It's the definition of IQ. 15 points is 1 standard deviation, so about 15% of the population has an IQ of 85 or below by definition.

Just following up to add that these two 15's are unrelated, in case any readers get the wrong idea.

The area under the normal distribution curve from minus infinity to -1 (one standard deviation left of centre) is about 0.1586 or about 15-16%.

The 15 points = 1 stddev property of IQ is arbitrarily established.