Scientists are learning how the brain chooses what to remember and forget.

“The man with a clear con-science probably has a poor memory,”an old adage says. Indeed, most people are occasionally pestered with recollections of past losses, blunders and slipups that bubble up from the depths of memory to nag their way into present thoughts.

Would it not be a relief if we could simply expunge every embarrassing gaffe, or, more seriously, outright trauma from our minds? That might seem tempting. But then, without memories, even the bad ones, who exactly are we?

Some recent Times articles explore the essential nature of memories, how they change with time, how they shape identity, and what life would be like without them.

The idea of excising bad memories is not as far-fetched as it might seem. As Benedict Carey reported in The Times, university researchers in Brooklyn, New York, have demonstrated on rats a drug that can block a chemical that is critical in memory. If delivered to specific areas of the human brain, it might erase chronic fears or even addictions.

The risk, Mr. Carey wrote, would be if people also lost“other, personally important memories that were somehow related.”

A cautionary tale might be found in the life of Henry Molaison, who died late last year at a nursing home in Connecticut, at age 82. When Mr. Molaison was 27, doctors operated on his brain to relieve seizures caused by a head injury.

Unfortunately, the surgery left him with profound amnesia. As Mr. Carey wrote in a Times obituary,“for the next 55 years, each time he met a friend, each time he ate a meal, each time he walked in the woods, it was as if for the first time.

Though he became an object of intense scientific interest for the rest of his life, the change was profound.

“Say it however you want,”Dr. Thomas Carew, a neuroscientist, told Mr. Carey.“What H.M. lost, we now know, was a critical part of his identity.”

But for those who do go through life with bad memories, there is good news. M.R.I. scans indicate that as people age, the part of the brain devoted to negative memories shifts. In young people, these memories are processed in a clump of brain tissue devoted to feelings, but in older people they arise from a center of rational thought.

Florin Dolcos, an assistant professor of psychiatry at the University of Alberta in Canada, told The Times’s Nicholas Bakalar that as a result older people are“better able to control these emotions, and this control influences their memory for negative information.”

Filtering memories with the passage of time has other advantages, as The Times’s John Tierney reported. Studies disclose that though people may feel bad in the short term about an expensive purchase or a hedonistic holiday, eventually they may congratulate themselves.

“People feel guilty about hedonism right afterwards,”Ran Kivetz, a professor of marketing at the Columbia Business School, told Mr. Tierney.“At some point there’s a reversal, and what builds up is this wistful feeling of missing out on life’s pleasures.”

So if you are tempted to overspend, overindulge or overreach, don’t worry about how you will feel the next morning. Consider, instead, the next decade.