London: Human brains can't get any smarter because they seem to have hit an evolutionary roadblock. You can't have too much of a good thing when it comes to mental performance because there is an upper limit somewhere, researchers suggest.

"In other words... if you go beyond that spot, just like in the fairy tales, you have to pay the price," says Thomas Hills from the University of Warwick in Britain.

"These kinds of studies suggest there is an upper limit to how much people can or should improve their mental functions like attention, memory or intelligence," said Hills, the journal Current Directions in Psychological Science reports.

"Take a complex task like driving, where the mind needs to be dynamically focused, attending to the right things such as the road ahead and other road users - which are changing all the time," he said, according to a university statement.

"If you enhance your ability to focus too much, and end up over-focusing on specific details, like the driver trying to hide in your blind spot, then you may fail to see another driver suddenly veering into your lane from the other direction," he added.

This seems to explain why people with enhanced intelligence suffer from autism, debilitating synaesthesia and neural disorders.

Hills, who conducted a comprehensive review study with Ralph Hertwig from the University of Basel in Switzerland, said drugs like Ritalan only help people with lower attention spans.

But when people who don't have trouble focusing, take these attention boosting pills, they can actually perform worse.

"Or if you drink coffee to make yourself more alert, the trade-off is that it is likely to increase your anxiety levels and lose your fine motor control. There are always trade-offs," Hills concluded.