To test the theory, researchers gave 132 participants one of three tasks: using tweezers to remove small pieces from slots in the Japanese equivalent of the board game Operation, counting the appearances of a digit in a block of numbers, and a reaction time test.

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Mexican officials put the successes down to better intelligence and co-operation with America, plus a fivefold increase in the size of the federal police force (counting officers seconded from the army).

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