GWEN IFILL: New research suggests there may be some good news in the struggle against dementia.

Two recently released studies show severe memory loss declining among healthier and better educated populations. In England and in Wales, dementia rates over the last two decades have dropped by 25 percent among those 65 and older.

And in Denmark, the percentage of elderly whose cognitive abilities were severely impaired also dropped between 1998 and 2010.

In the United States, about five million people have Alzheimer's disease, but that number is expected to rise sharply as baby boomers age.

For more, we're joined by Dr. Murali Doraiswamy, director of the Neurocognitive Disorders Program at the Duke University School of Medicine.

Thank you for joining us.

DR. MURALI DORAISWAMY, Duke University School of Medicine: Thank you very much. Great pleasure to be on.

GWEN IFILL: What are the most hopeful signs you see in these new studies?

MURALI DORAISWAMY: Well, this is terrific news.

The so-called silver tsunami that we have all been scared of has just downgraded from grade five to grade four.

So, the key thing to keep in mind is we're not out of the woods, but what these two studies are telling us is that successive generations or even slightly younger cohorts separated by as little as 10 years apart may not have the same risk.

So in other words, our children or our grandchildren may not have the same risk for Alzheimer's that we do. The second thing I think that these studies are pointing out is if the risk for Alzheimer's is going down with successive generations, then that is good news because it indicates that it is likely to be due to environmental or lifestyle effects.

In other words, many of the public health interventions that have been put into place since the 1970s, such as encouraging Americans and people all over the world to exercise more, cutting down on smoking, the disappearance of the Marlboro Man, if you will, eating healthier, and indeed better education, I think all of these things might be having an effect.

GWEN IFILL: But do you see any red flags in this research? We're talking about England and Wales. They're not necessarily the same type of population as we see here in the U.S., for instance.

MURALI DORAISWAMY: That's not the main red flag.

There are studies that have been done in Sweden, in the U.S., in many countries that show the same essential decreasing incidence rates, if you will, for Alzheimer's in successive generations.

That said, these are also called observational studies, so these are not clinical trials, where people are sort of randomly assigned to different treatment arms. So, we cannot be sure, but I think the signs from all these different studies in multiple countries all are pointing in the same direction.

GWEN IFILL: But at an Alzheimer's meeting conference today, they were talking about memory loss being more of a warning sign than we have been necessarily led to believe. We have been told, well, losing your keys is not necessarily a problem, but now people are saying maybe it is. Help us with that.

MURALI DORAISWAMY: Well, memory problems have always been a warning sign of Alzheimer's.

I think what scientists are discovering now is that Alzheimer's can often start in a very, very mild form called a subjective cognitive impairment. These are not the so-called benign senior moments where you forget your key or you occasionally have a tip-of-the-tongue problem.

I think what we're talking about are much more serious memory problems, where people are not even remembering what they forgot. They're having trouble planning. They are forgetting the names of loved ones and those names never come back.

If you forget something and it comes back to you a couple of hours later, it's probably benign. But, that said, this research is still in early days, but I think the key point to remember is that there could be many, many causes of memory loss as you get older. If your memory problems start affecting your daily functioning, then that's when you need to take it more seriously.

And it doesn't always have to be Alzheimer's, because a number of conditions can mimic Alzheimer's, and many of them are reversible, such as depression or vitamin deficiencies or thyroid problems.

GWEN IFILL: And there are other measures of brain health that you use to make these determinations, just besides diet or other health measures, right?

MURALI DORAISWAMY: Absolutely.

When we see someone coming in with mild memory complaints, we run the full battery of laboratory tests. We also do formal neuropsychological testing. And sometimes we have to get brain scans to make sure that it's not a small stroke or some other lesion in the brain that's causing these.

But for the vast majority of people with mild forgetfulness, I don't think they have anything to worry about.

GWEN IFILL: How do these stories square with -- we -- it feels like every six weeks, six months, we do another story on this program about another study.

How does this square about the one we heard that in the next 30 years, Americans with Alzheimer's will double? Are we talking about people who are alive now, as opposed to our children? Is it a question of scope?

MURALI DORAISWAMY: Well, so, first of all, you have to realize that many of these forward projections are merely estimates, and some of the previous projections fail to take into account the fact that with better health improvements, with reduction of cardiovascular disease, with reduction of smoking, with more exercise, the rates of Alzheimer's may actually go down in the future.

So, many of these projections are assuming that the rates are going to hold steady or are going to go up simply because of the rise in the older population.

So, the bottom line is public officials now probably have to revise some of these estimates a little bit lower, so, it's good news, but we're not entirely out of the woods.

GWEN IFILL: Now, dementia is a much broader umbrella than Alzheimer's specifically. So, if there's good news on dementia, does that always mean -- also mean there's good news on Alzheimer's and more specific cases?

MURALI DORAISWAMY: Most of the time, but not always.

It could very well be that much of the reductions that we're seeing are reductions in a type of dementia calls vascular dementia, which is accounted largely by cardiovascular disease, by strokes, by high blood pressure, by high cholesterol.

I suspect that that is the area where we have made the biggest gains, because we now have better ways to treat cardiovascular disease. I suspect that Alzheimer's has also gone down a little bit because we're getting better at education. And Alzheimer's is also linked to heart disease, even though not as strongly as vascular dementia.

GWEN IFILL: But we don't know for sure that the rate yet -- we don't have any reliable studies that show that the rate of Alzheimer's is declining, like the rate of dementia, as we see in these studies?

MURALI DORAISWAMY: So, I think what we have to differentiate is the risk vs. the total numbers.

The total numbers of Alzheimer's disease are going to climb upwards quite dramatically because older age and the rising number of people who live into their 70s, and 80s and 90s is a huge risk factor for Alzheimer's.

But within each generation, what we're seeing is a given person's risk for developing Alzheimer's might actually reduce and go down.

GWEN IFILL: Dr. Murali Doraiswamy of Duke University, thank you very much for helping us out.

MURALI DORAISWAMY: Thank you very much.