The brain has trouble with certain types of calculations, but not with ones that involve social issues. The reason is that the brain isn't so familiar with the rules of conditional logic, but it's good with social issues.
Imagine that you were shown four cards with the following numbers and colours: 7, BROWN, 10, BLUE. Here is a claim: If a card has an even number on one side, it has the name of a primary colour on the opposite side. Which two cards do you need to turn over to assess whether or not this is the truth?
For many people this is a difficult problem. The answer is that we need to turn over the number 10 card and the brown card. If we turned over the number 7 card and found blue on the other side, it would say nothing about the truth of the rule. This is because the statement only concerns even-numbered cards. Likewise, if we turned over the blue card and found an odd-numbered card on the other side, it would make no difference to the logical rule because it never specified what odd numbers may have on the other side.
But if we present the same problem with a social issue, it's solved easily. Suppose this is the rule: If you're under 18, you can't drink alcohol. Now each card has the age of one person on one side and the drink the person is holding on the other: RUM, 35, COKE, 16. Which cards do we need to turn over to see if the rule is being broken? Here most participants get the answer right. We need to turn over RUM and 16. The two problems are equivalent, but this one is easier to solve. People of any age can drink coke, but only those 18 and over can drink rum. People who are 35 can drink coke and rum, but those who are 16 can only drink coke.
The brain cares so much about social interaction that it has evolved special programs devoted to it. When a problem concerns a social issue such as the legal drinking age, participants can solve a logical problem easily. This is not true when the issue concerns number and colour. It appears that the brain is better conditioned to solve problems of social interaction than equivalent problems without social interaction.