The reason we struggle to recall memories from our early childhood is down to high levels of neuron production during the first years of life, say Canadian researchers.
The formation of new brain cells increases the capacity for learning but also clears the mind of old memories.
This could be behind the absence of long-term memory events from early childhood, known as infantile amnesia.
The findings were presented to the Canadian Association of Neuroscience.
Neurogenesis, or the formation of new neurons in the hippocampus - a region of the brain known to be important for learning and remembering, reaches its peak before and after birth. It then declines steadily during childhood and adulthood.
Dr Paul Frankland and Dr Sheena Josselyn, from the Hospital for Sick Children in Toronto and the University of Toronto, wanted to find out how the process of new neuron generation impacted on memory storage.
They carried out their research on younger and older mice in the lab.
In adult mice, they found that increasing neurogenesis after memory formation was enough to bring about forgetting.
In infant mice, they discovered that decreasing neurogenesis after memory formation meant that the normal forgetting observed at this age did not occur.
Their research suggests a direct link between a reduction in neuron growth and increased memory recall.
They found the opposite to be true also - a decreased ability to remember when neurogenesis is increased (as happens during infancy).
The researchers said this provided an explanation for the absence of long-term memory events from early childhood, known as infantile amnesia.
Previous studies have shown that although young children can remember events in the short term, these memories do not persist.
Dr Frankland, senior scientist in neuroscience and mental health, at the hospital, said: "Why infantile amnesia exists has long been a mystery.
"We think our new studies begin to explain why we have no memories from our earliest years.
"Before the ages of four or five, we have a highly dynamic hippocampus which can't stably store information.
"As new neurons are generated, memory may be compromised by that process."
Dr Bettina Forster, from the cognitive neuroscience research unit at City University in London, said the research showed a clear link.
"This is a very interesting and elegantly executed study which shows a direct link between neurogenesis and memory formation.
"The results questions the long assumed link between verbal development and infantile amnesia and calls into question some psychological and psychotherapeutic theories on this topic."