In general, adults do not remember events that took place prior to age 3. Psychologists refer to this inability of most adults to remember events from early life, including their birth, as childhood amnesia. Sigmund Freud first coined the term infantile amnesia, now more broadly referred to as childhood amnesia, as early as 1899 to explain his adult patients' scarcity of childhood memories. Freud proposed that people use it as a means of repressing traumatic, and often sexual, urgings during that time. To block those unconscious drives of the id, Freud claimed that humans create screen memories, or revised versions of events, to protect the conscious ego.

More than a century later, researchers have yet to pin down a precise explanation for why childhood amnesia occurs. Only in the last 20 years have people investigated children's, rather than adults', memory capabilities in search of the answer. This research has brought with it a new batch of questions about the nuances of young children's memory.

For a long time, the rationale behind childhood amnesia rested on the assumption that the memory-making parts of babies' brains were undeveloped. Then, around age 3, children's memory capabilities rapidly accelerate to adult levels.

However, psychologists have discovered that children as young as 3 months old and 6 months old can form long-term memories. The difference comes in which memories stick around. For instance, it appears that babies are born with more intact implicit, or unconscious, memories. At the same time the explicit, or episodic, memory that records specific events does not carry information over that three-year gap, explaining why people do not remember their births.

But why does this happen, and what changes take place in those first years? And if we can form memories as babies, why don't we retain them into adulthood? On the next page, we'll take a closer look at a baby's brain to find out the answer.

To form memories, humans must create synapses, or connections between brain cells, that encode sensory information from an event into our memory. From there, our brains organize that information into categories and link it to other similar data, which is called consolidation. In order for that memory to last, we must periodically retrieve these memories and retrace those initial synapses, reinforcing those connections.

Studies have largely refuted the long-held thinking that babies cannot encode information that forms the foundation of memories. This memory encoding could relate to a baby's development of the prefrontal cortex at the forehead. This area, which is active during the encoding and retrieval of explicit memories, is not fully functional at birth. However, by 24 months, the number of synapses in the prefrontal cortex has reached adult.

Also, the size of the hippocampus at the base of the brain steadily grows until your second or third year. This is important because the hippocampus determines what sensory information to transfer into long-term storage.

But what about implicit memory? Housed in the cerebellum, implicit memory is essential for newborns, allowing them to associate feelings of warmth and safety with the sound of their mother's voice and instinctively knowing how to feed. Confirming this early presence, studies have revealed few developmental changes in implicit memory as we age. Even in many adult amnesia cases, implicit skills such as riding a bicycle or playing a piano often survive the brain trauma.

Now we know that babies have a strong implicit memory and can encode explicit ones as well, which indicates that childhood amnesia may stem from faulty explicit memory retrieval. Unless we're thinking specifically about a past event, it takes some sort of cue to prompt an explicit memory in all age groups.

Our earliest memories may remain blocked from our consciousness because we had no language skills at that time. A 2004 study traced the verbal development in 27- and 39-month old boys and girls as a measure of how well they could recall a past event. The researchers found that if the children didn't know the words to describe the event when it happened, they couldn't describe it later after learning the appropriate words.

The act of recognizing oneself in the mirror is one of the first indicators of developing autobiographical memory. Verbalizing our personal memories of events contributes to our autobiographical memories. These types of memories help to define our sense of self and relationship to people around us. Closely linked to this is the ability to recognize yourself. Some researchers have proposed that children do not develop self-recognition skills and a personal identity until 16 or 24 months.

In addition, we develop knowledge of our personal past when we begin to organize memories into a context. Many preschool-age children can explain the different parts of an event in sequential order, such as what happened when they went to a circus. But it isn't until their fifth year that they can understand the ideas of time and the past and are able to place that trip to the circus on a mental time line.

Parents play a pivotal role in developing children's autobiographical memory as well. Research has shown that the way parents verbally recall memories with their small children correlates to those children's narrative style for retelling memories later in life. In other words, children whose parents tell them about past events, such as birthday parties or trips to the zoo, in detail will be more likely to vividly describe their own memories. Interestingly, autobiographical memory also has a cultural component, with Westerners' personal memories focusing more on themselves and Easterner's remembering themselves more in group contexts.

 (Contributed by Teddy R.)