A new study may show why we cannot recall the very earliest parts of life.

For years the mysterious lost memories from early childhood have confused experts, but new research suggests they may have been wiped out by the growth of new brain cells.

A study in rodents suggests infants’ memories may be wiped clean to make room for new ones, as brain cells are generated throughout the first stage of life.

The findings may help solve a question that has been asked for decades.

Sigmund Freud said that ‘infantile amnesia’ may have a psychosexual origin, later scientists speculated that memories might not become long-term until the development of language, but the recent findings suggest that the forgetful effect may be linked to brain cells themselves.

“The really weird thing is that most animals show infantile amnesia too,” says study coauthor Sheena Josselyn.

“So the development of language can’t be the whole explanation.”

Memories of situations and events come through the hippocampus, a smalle belt of tissue stretching from ear to ear. The hippocampus also houses a brain cell factory just a few centimetres across. It is the only part of the brain that produces new neurons, but their production seems to taper off in childhood.

“That’s exactly when we start to be able to form long-term memories,” Josselyn says.

To test the link between recollections and brain cell formation, the researchers placed adult mice in a chamber noticeably different from their usual homes – with distinct stripes on the walls and a strong smell of vinegar - and buzzed the animals with mild shocks to the feet. The mice learned to fear the room, and even 28 days later would freeze up when put in the chamber.

A day after being shocked, the fear in infant mice began to fade, suggesting they were more forgetful.

Next, researchers boosted neuron production (neurogenesis) in adult mice, and observed after a few weeks that the mice forgot their fear of the scary room.

The reverse worked too; keeping the birth of new neurons to a minimum in infant mice kept the fear memory alive.

“It was really amazing to us that we could make a memory last much longer in these infant mice just by decreasing neurogenesis,” Josselyn said.

The notion is “contradictory to where everyone else in the field has been,” co-author, neuroscientist and psychiatrist Thomas Insel said.

“That’s going to be very provocative.”

“Maybe forgetting is not a bad thing,” Josselyn says.

“Maybe it’s good to clear away some memories and forget some things that are not so important.”