disadvantage of being smart

Actually, less intelligent people are better at doing most things. 

evolution equipped humans with solutions for a whole range of problems of survival and reproduction. All they had to do was to behave in the ways in which evolution had designed them to behave—eat food that tastes good, have sex with the most attractive mates.  

General intelligence is very important in modern life because our environment is almost entirely evolutionarily novel. Most of the problems that we have to solve today—how to excel in school, how to find jobs, how to do virtually everything on a computer—are evolutionarily novel. So intelligent people do well in almost every sphere of modern life, except for the most important things, like how to find a mate, how to raise a child, how to make friends. Intelligence does not confer any advantage for solving all the evolutionarily familiar problems that our ancestors encountered. More intelligent people do not have any advantage in finding mates and often have disadvantages.

General intelligence evolved to solve evolutionarily novel problems, so intelligent people are more likely to acquire and espouse evolutionarily novel preferences and values. They are more likely to recognise and develop tastes for things that our ancestors did not have 100,000 years ago. For example, more intelligent people are more likely to be left-wing liberals because our ancestors were “conservative” by the contemporary American definition—they only cared about the well-being of their friends and family. They are more likely to be atheist because the preferred theory in evolutionary psychology is that humans are designed to believe in God.

Humans appear to be designed to be paranoid; they are designed to see intentional agents behind natural phenomena. This is because making the mistake of thinking that a natural event has an intentional agent behind it is less potentially costly than being oblivious and thinking that an intentional event, like someone trying to kill you, has a coincidental cause. The paranoid outlive the oblivious. Belief in God may be a consequence of this tendency. Intelligent people are more likely to be nocturnal because humans are designed to wake up when the sun comes up and go to sleep when the sun goes down. They are more likely to be homosexual, because humans are evolutionarily designed to reproduce heterosexually. They are more likely to enjoy instrumental music because music in its evolutionary origin was vocal, and they are more likely to consume alcohol, cigarettes and drugs because all of these substances are evolutionarily novel.  

Some people suggest that more intelligent people think too much and therefore need alcohol to stop thinking, but that's not my argument. My point is that the human consumption of alcohol, tobacco and psychoactive drugs is a relatively new phenomenon. Both the American and British population data (nationally representative samples of Americans and Brits) show that more intelligent people consume more alcohol more frequently.

Would you rather be a good brain surgeon or a good parent? Would you rather be a good corporate executive or a good friend? More intelligent people don't always make good parents or friends. Intelligent women make the worst kind of parents, simply because they are less likely to become parents in the first place. There is also some evidence that children of more intelligent women are more likely to suffer from health and behavioural problems, probably due to the fact that they tend to have children later.

Why?

Because reproductive success is the ultimate goal of all living organisms, so intelligent women are more likely to go against such evolutionary design. My theory would also predict that intelligent men should be less likely to become parents, but data do not confirm that. Some suggest that women prefer to have children with more intelligent men, but the data contradict this too. Men's income or education has no effect on their likelihood of becoming parents. Intelligence doesn't allow us to do better what we are designed by evolution to do. Saint Exupery writes; “Voici mon secret.  Il est très simple: on ne voit bien qu'avec le cœur.  L'essential est invisible pour les yeux.”