Researchers found a particular chemical, or neurotransmitter, known as Gaba, held the key.
Gaba is the brain's main "inhibitory" neurotransmitter. That means, when it's released by one nerve cell it suppresses the activities of other cells to which it is connected.
They found people who had the highest concentrations of Gaba in their brain's hippocampus (or memory hub) were best at blocking unwanted thoughts or memories.
"What's exciting about this is that now we're getting very specific," said Prof Anderson.
"Before, we could only say 'this part of the brain acts on that part', but now we can say which neurotransmitters are likely to be important."
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