No doubt Charles R. Darwin was right when he discovered 'the survival of the fittest', even Sharon Moalem has an excellent point when he speaks about 'the survival of the sickest' , but what is really shocking is what I recently discovered: 'The Survival of the Stupidest'.
Here is a way to look at it:
Our brains are constantly integrating information about the world around us. That requires neuronal activity that can not be used at the same time to to do other things, such as actually think. Multi tasking is a myth debunked time and again .
Deep thinking about truly complex problems require total concentration. There is no room for assessing ones environment for imminent threats. So if in the early days of the hominoids, at the dawn of homo sapiens, our species would have been engaged in deep thinking the result would be clear: the individual would simply not survive long enough to reproduce. Consequently there will be no 'deep thought' genes to be inherited .
... and then there is a Sabre-Tooth Cat lurking in the back ...
This selective process continues to date: try thinking very hard about a complex problem while crossing the road... If you start young enough doing so you can rest assured that your genes will stay unique and will not be transferred into the pool of humanity.
And because of the pre-selection that already happened over the past couple of hundred thousand years, there can only be one conclusion: We are stupid.
 Sharon Moalem, 'Survival of the Sickest', HarperCollins Publishers, 2007.
 The Myth of Multi Tasking: https://www.psychologytoday.com/blog/creativity-without-borders/201405/the-myth-multitasking
 Of course it is still an open debate whether 'deep thought' genes actually exist, or whether it comes about from epigenetic selection or is just adaptation or nurturing...