There is plenty of long-standing, well established documented evidence that genetics is the largest portion controlling IQ.

Also, there are simple IQ tests such as repeating a sequence of letters or numbers backwards that is independent of culture. "Cultural differences" in testing was looked at decades ago, in the hope that it would create "equity."

There are things that can take away from IQ, such as traumatic brain injuries, birth defects, or lack of proper nutrition. However, we unfortunately haven't found "magic pills" that can measurably and consistently increase IQ.

Years ago it was believed that certain racial groups would increase their average IQ as they became more prosperous, had better schools, had affirmative action, or were free. Sadly, that hasn't happened. In fact, overall IQ in the US is going down, not up, and it is also happening in many European/Western nations. The average IQ's of those "disenfranchised" groups gained in the early 20th century, but have now either flatlined or regressed.

Sports gives us an obvious and unbiased demonstration that genetic differences are real, persistent and scientific. It should be no surprise that genetics are also a large factor in mental abilities as well.