IQ is not equally distributed across the human population, and we are constantly discovering new genes which are responsible for fractions of it (or the opposite - aggressiveness and violence).

If you move someone from say Sudan, with an average IQ of 70, to Sweden, with an average IQ of 99, then the average in Sweden will fall.

https://www.worlddata.info/iq-by-country.php

IQ being unequally distributed makes sense when you consider that population groups have been separated for 10s of thousands of years and exposed to very different selective pressure, and the unequal level of achievement of different human groups historically and currently.

This is also precisely why high-IQ couples should have children - to pass their valuable genetics and culture (to describe the high-quality upbringing that they could provide) to the next generation.

Fortunately genetic engineering and molecular biology is advancing rapidly, where we can identify specific genes related to behavioural traits eg: https://en.wikipedia.org/wiki/Monoamine_oxidase_A and eventually add and remove certain genes, or at least select certain embryos, based on desired changes.

Richard Lynn has also written extensively on the topic: https://en.wikipedia.org/wiki/IQ_and_Global_Inequality