It is true that genes don’t determine intelligence completely; they only do so substantially and profoundly.

Heritability is the measure of the influence of genes on any trait that vary between individuals in a population.  Heritability of 1.0 means that genes determine the traits completely and the environment has absolutely no effect.  Some genetic diseases like Huntington’s disease have heritability of 1.0; genes entirely determine whether or not you will get Huntington’s disease.  If you have the affected gene for the disease, it does not matter at all how you live your life or what your environment is; you will develop the disease.  One’s natural eye color or natural hair color also has heritability of 1.0.  So does one’s blood type.  Very few other human traits have heritability of 1.0.

On the other hand, heritability of 0 means that genes have absolutely no influence on a given trait, and the environment completely determines whether or not someone has the trait.  No human traits have heritability of 0; genes partially influence all human traits to some degree.  (This is known as Turkheimer’s first law of behavior genetics.)

Most personality traits and other characteristics – like whether you are politically liberal or conservative or how likely you are to get a divorce – have heritability of .50; they are about 50% determined by genes.  In fact, most personality traits and social attitudes follow what I call the 50-0-50 rule; roughly 50% heritable (the influence of genes), roughly 0% what behavior geneticists call “shared environment” (parenting and everything else that happens within the family to make siblings similar to each other), and roughly 50% “nonshared environment” (everything that happens outside of the family to make siblings different from each other).  It turns out that parenting has very little influence on how children turn out.

Of course, this emphatically does not mean that parents are not important for how children turn out; they are massively and supremely important because children get their genes from their genetic parents.  It simply means that parenting – how parents raise their children – is unimportant.  This is why adopted children usually grow up to be nothing like their adoptive parents who raised them and a lot like their biological parents (or their identical twins reared apart) that they have never even met.

One of the very few exceptions to the 50-0-50 rule is intelligence, for which heritability is larger.  Heritability of general intelligence increases from about .40 in childhood to about .80 in adulthood.  Among adults, intelligence is about 80% determined by genes.

Yes, heritability of intelligence increases over the life course, and genes become more important as one gets older.  This may at first sight seem counterintuitive, but it really isn’t.  This is because for adults the environment is part of their genetic makeup whereas for children it isn’t.  Children must live in the environment created by their parents, older siblings, teachers, neighbors, clergy, and other adults.  In contrast, adults determine their own environment to a much greater extent than children do.  So for adults, genes and the environment become more or less the same thing, whereas for children they are not.  For adults, when the environment influences their intelligence, it shows up as the influence of their genes, which largely determine their environment, whereas for children it does not.  This is why the influence of genes increases dramatically throughout life.

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Read more about the surprising facts about intelligence, what it is (and what it is not), how it affects you in virtually every sphere of life, and how more intelligent people are different from less intelligent people, in The Intelligence Paradox:  Why the Intelligent Choice Isn’t Always the Smart One.