At the individual level we know that it can be very difficult to unlearn bad habits. For example, a once learned but non-optimal fingering on the piano is almost impossible to unlearn. The problem, as Dickens describes so beautifully in his famous Christmas Carol, is that we indeed make the chain we forge in life link by link. Certain habits and addictions are sometimes so difficult to leave behind. The force of habit. Brain rule number 1 - the Hebbian rule - says Cells that fire together wire together. And what applies to cells in our brain also applies to "aggregated" levels: people who join together form each other.