The Economist explains learning without learning — via epigenetic imprinting:
There is a growing school of thought that Freud was right, but for the wrong reasons. According to the members of this school, early experience does profoundly mould the brain. However, it is not memory that it moulds — at least, not memory as conventionally understood. What it actually moulds is the way genes work.
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The first inkling of this came when Michael Meaney, one of Dr Szyf’s long-term collaborators, noticed that rat pups whose mothers spent a lot of time licking and grooming them grew up to be less fearful and better-adjusted adults than the offspring of neglectful mothers. Crucially, these well-adjusted rats then gave their own babies the same type of care — in effect, transmitting the behaviour from mother to daughter by inducing similar epigenetic changes.When Dr Szyf looked at the brains of the two sorts of rats, he found differences in their hippocampuses. Among other jobs, the hippocampus is involved in responding to stress. Dr Szyf discovered that better-adjusted rats had, in their hippocampuses, more active versions of the gene that encodes a molecule called glucocorticoid-receptor protein. Glucocorticoid is a hormone produced in response to stress and its job is to make the animal behave appropriately. But too much glucocorticoid is a bad thing, so there is also a way to switch off its production. When glucocorticoid binds to its receptor in the hippocampus, that activates the expression of genes which dampen further synthesis of the hormone. This feedback system is weaker in rats that have had little maternal care. As a result, they are more anxious and fearful, and show a heightened response to stress.
The researchers went on to study what is responsible for the difference in expression of the glucocorticoid-receptor gene. They found that two types of imprinting are involved. One adds molecules called methyl groups to the DNA of the gene. This suppresses gene expression. The other adds acetyl groups, which are slightly larger, to the proteins around which the DNA is coiled. This has the opposite effect, making gene expression easier. Rats that had experienced little maternal care showed high levels of methylation and low levels of acetylation of the glucocorticoid-receptor gene and its neighbouring proteins. The opposite was true for those that had had a more attentive upbringing.