A smart person with a burning desire to produce offspring will have the greatest advantage. A smart person who dislikes children will have the greatest disadvantage. In between those people are the people who screw up birth control.
If the human population contains a million people with the trait for wanting lots of kids and a billion people who are just dumb about birth control, both will be selected by evolution. Once those traits are the norm, it would likely turn out that wanting kids is the more successful strategy. We could thus see intelligence go both directions, first dropping and then rising.
Since then the useful selective function for individual survival has been the ability to operate in a tribe and make the tribe successful.
Who's to say what that has really been selecting for? Intelligence may very well have almost no correlation with individual genes, and simply be an emergent phenomenon optimized for by pro-social behaviors, or activated by being in the presence of people who are already fulfilling that role. Maybe what we regard as individual intellect is repressed pathways only chemically triggered in childhood after the development cycle casts around to try and figure out the status of the tribe and specialize into the optimum addition?
50% of what? (which is to say, you picked that number because 50% genetics 50% environment feels like a good split, and how do you even measure that?)
It means that if you plotted a graph of orphaned children IQs on the X axis and their biological parents IQs on the Y axis you'll get a messy and very bulgy line going up and to the right.
And this kind of thing is discovered by noticing that the IQs of identical twins are much more similar than those of non-identical twins.
Wikipedia has a fairly good overview: https://en.m.wikipedia.org/wiki/Heritability_of_IQ
Huh, guess I missed the news that we'd discovered human level problem solving abilities in sheep.
People with lower intelligence may be just as likely to adopt a short term hedonic lifestyle and use contraception more. Totally depends on cultural trends.
What we are selecting for is any heritable trait that might durably make people more likely to intentionally reproduce. That could be a lot of things: a stronger biological clock, later menopause for women, changes in cognitive patterns to the degree that they are heritable, maybe even longevity or longer healthspan. Its complex and hard to predict.