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The Seattle Longitudinal Study of Adult Intelligence

The Seattle Longitudinal Study of Adult Intelligence has followed a group of more than 5000 people for well over four decades. The program began in 1956 and participants have been tested across a whole gamut of mental and physical abilities at seven year intervals since that date.

The study has found:

  • no uniform pattern of age-related change across all intellectual abilities
  • some support for the idea that abilities that are primarily genetically determined tend to decline earlier than abilities that are primarily acquired through schooling or experience (although there may be gender differences here)
    • although abilities that are primarily genetic may decline earlier, abilities acquired through training decline more steeply after late 70s the change in perceptual speed begins in young adulthood and declines in a linear fashion (that is, the rate of decline is constant)
    • the rate and magnitude changes in intelligence seen in those entering old age showed greater decline in the 1st 3 cycles (till 1970); at the same time, younger members are scoring lower on tests at the same age.
    • a decline in psychometric abilities is not reliably observed before 60, but is reliably observed by 74. However, even by 81, fewer than half showed reliable decrements over the past seven years.
    • the size of this decline however is significantly reduced when age changes in perceptual speed are taken into account.
    • substantial cohort / generational differences have been observed. Later-born groups have attained successively higher scores at the same ages for inductive reasoning, verbal meaning, and spatial orientation; however, they’ve scored successively lower in number skill and word fluency (number skill peaked with the 1924 cohort). These changes presumably reflect educational changes.
    • substantial similarity between parents and their adult children and between siblings has been found for virtually all mental abilities and measures of flexibility (the exceptions are the attitude measure of social responsibility, and a measure of perceptual speed). The magnitude of similarity varied for different abilities, and was closer between parent & child than between siblings.
    • the following variables may reduce the risk of cognitive decline in old age:
      • absence of chronic diseases
      • a complex and intellectually stimulating environment
      • a flexible personality style at mid-life
      • high intellectual status of spouse
      • maintenance of high levels of perceptual processing speed
    • cognitive training studies suggested that the observed decline in many community-dwelling older people is probably a function of disuse and is often reversible. Some 2/3 of participants in a cognitive training program showed significant improvement, and 40% of those who had declined significantly were indeed returned to their earlier (pre-decline) level of cognitive functioning. These training gains were retained over seven years.


  1. Schaie, K. Warner 1998. The Seattle Longitudinal Studies of adult intelligence. In M. Powell Lawton & Timothy A. Salthouse (eds) Essential papers on the psychology of aging. NY: NY Univ Pr. Pp263-271.

Mild Cognitive Impairment

Except in the cases of stroke or traumatic brain injury, loss of cognitive function is not something that happens all at once. Cognitive impairment that comes with age may be thought of as belonging on a continuum, with one end being no cognitive impairment and the other end being dementia, of which Alzheimer's is the most common type.

Most older adults are actually at the "no impairment" end of the continuum. A further 30-40% of adults over 65 will have what is called "age-related memory impairment", which is the type of cognitive loss we regard as a normal consequence of age -- a measurable (but slight) decline on memory tests; a feeling that you're not quite as sharp or as good at remembering, as you used to be.

Only about 1% of these people will develop Alzheimer's.

But around 10% of adults over 65 develop "mild cognitive impairment", and this is a precursor of Alzheimer's. This doesn't mean someone with MCI will inevitably get Alzheimer's in their lifetime, but their likelihood of doing so is substantially increased.

Whether you are one of those 10% depends in part on your age and your level of education. A study2 of nearly 4000 people from the general population of a Minnesota county, run by the Mayo Clinic, indicates 9% of those aged 70 to 79 and nearly 18% of those 80 to 89 have MCI. The prevalence decreased with years of education: it was 25% in those with up to eight years of education, 14% in those with nine to 12 years, 9% in those with 13 to 16 years, and 8.5% in those with greater than 16 years.

Whether or not this will develop into Alzheimer’s can be predicted with a reasonably high level of accuracy (75%) by the rate at which brain tissue is being lost, and in particular the rate at which it is being lost in the hippocampus (arguably the most important region for memory in the brain). Whether actions known to build brain tissue (physical exercise, mental stimulation) can counteract that in this population is not yet known — but it certainly can’t hurt!

Mild cognitive impairment doesn’t necessarily mean memory problems. There are two types of MCI: those with the amnesic subtype (MCI-A) have memory impairments only, while those with the multiple cognitive domain subtype (MCI-MCD) have other types of mild impairments, such as in judgment or language, and mild or no memory loss. Both sub-types progress to Alzheimer's disease at the same rate, but they do have different pathologies in the brain.

Mild cognitive impairment is not necessarily obvious to outside observers. A person with it can function perfectly well, and although they may feel their impairment is obvious to all around them, it's not likely to be obvious to anyone not living with them.

A person suffering from mild cognitive impairment may find that they have problems with:

  • finding the right words
  • making decisions
  • remembering recent events
  • placing things in space (for example, getting the proportions right when drawing a simple object such as a box).

Essentially, age-related cognitive impairment might be thought of as slight, non-important, cognitive impairment, while mild cognitive impairment is a condition where significant cognitive impairment exists which nevertheless doesn't affect daily functioning. Dementia is significant cognitive impairment that does interfere with daily life.


See more research at my companion website About Memory

  1. Becker, J.T. et al. 2006. Three-dimensional Patterns of Hippocampal Atrophy in Mild Cognitive Impairment. Archives of Neurology, 63, 97-101.
  2. Petersen, R. et al. 2006. Study presented April 4 at the American Academy of Neurology meeting in San Diego. Press release
  3. Quinn, J.F. & Kaye, J.A. 2004. Study presented at the 56th annual meeting of the American Academy of Neurology in San Francisco. Press release
  4. Small, G.W. 2002.What we need to know about age related memory loss. British Medical Journal, 324, 1502-1505.

Memory in normal aging

People are poor at assessing their own memory

One thing research seems to show rather consistently is that, for older adults in particular, beliefs about one's own memory performance have little to do with one's actual memory performance¹. People who believe they have a poor memory are usually no worse at remembering than those who believe they have a good memory.

One theory for why this might be, is that people may be influenced by their general beliefs about how memory changes with age. If you believe that your memory will get progressively and noticeably worse as you get older, then you will pay more attention to your memory failures, and each one will reinforce your belief that your memory is indeed (as expected) getting worse.

Memory decline can be a self-fulfilling prophecy

Research has also shown that common, everyday memory failures tend to be judged more harshly when the failure belongs to an older adult². What is laughed off in a younger adult is treated as an indication of cognitive decline in an older person.

There are ways in which cognitive function (memory, reasoning, problem-solving, etc) declines with age, but it would be fair to say that general belief over-estimates the extent of this. It is, to a large extent, a self-fulfilling prophecy. If you believe deterioration is inevitable, you are not likely to make any effort to halt it.

Memory decline is associated with physical factors

A large-scale study that tracked seniors over a ten-year period found that cognitive decline is not a normal part of aging for most elderly people: 70% of the nearly 6000 seniors in the study showed no significant decline in cognitive function over the ten-year period. These people had two factors in common: they did not carry any of the apolipoprotein E4 genes (often associated with Alzheimer's disease), and they had little or no signs of diabetes or atherosclerosis³.Other factors that have also been implicated in age-related cognitive decline are obesity, smoking, and high blood pressure. Indeed, researchers have suggested that risk factors for cardiovascular disease are also risk factors for cognitive decline: what's bad for the heart is also bad for the brain.

  1. Hertzog, C. & Dunlosky, J. 1996. The aging of practical memory: an overview. in Herrmann, D.J., McEvoy, C., Hertzog, C., Hertel, P. & Johnson, M.K. (eds). Basic and applied memory research: Vol. 1:Theory in context. NJ: Lawrence Erlbaum.
  2. Erber, J.T., Szuchman, L.T & Rothberg, S. T. 1990. Everyday memory failure: Age differences in appraisal and attribution. Psychology & Aging, 5(2), 236-241.
  3. Haan, M.N., Shemanski, L., Jagust, W.J., Manolio, T.A. & Kuller, L. 1999. The Role of APOE4 in Modulating Effects of Other Risk Factors for Cognitive Decline in Elderly Persons. JAMA, 282, 40-46.

Why do some cognitive processes decline with age?

  • Most basic cognitive processes decline with advanced age at higher levels of difficulty.
  • Part of this reflects the slowing down that occurs with age.
  • It does seem likely however that there is a reduction in processing capacity with age.
  • Strategies that reduce the memory load of a task are therefore likely to be of help to older adults, for example:
    • Use of pictures as memory aids
    • Text that is clear and explicit
  • Practicing new skills and habit until they become automatic is also likely to be of even more help to older adults than younger adults (because it reduces memory load).
  • Irrelevant detail can be more distracting for older adults, and this may also play a part in cognitive decline.

Most cognitive processes decline with age

It does appear that most component processes of cognition decline with advanced age if the difficulty level is sufficiently high. For example, the following processes have all shown age effects:

  • processes involving attention
  • working memory capabilities (the amount of information you can work with without losing track of any)
  • understanding text
  • making inferences
  • encoding(putting information into memory) and retrieval (finding information in memory)

Other processes however, show little or no decline with age, for example:

  • picture recognition
  • implicit memory (information that can't be brought to mind but can be seen to affect behavior)
  • prospective memory (remembering things you need to do)

Additionally, older adults’ performance on highly practiced expert skills can match that of young adults (e.g., typing, bridge playing, chess).

Cognitive decline in normal aging mainly due to a reduced working memory capacity?

It would seem from this that cognitive decline in old age may be primarily due to the reduction in processing capacity - understanding text, making inferences, paying attention are all processes that depend heavily on your working memory capability. Accordingly, it has been theorized that cognitive aids that minimize the use of processing resources might be effective in helping older adults.

Since picture recognition is one of those cognitive processes that don't appear to be affected by age, pictures may well provide effective memory support for older adults. For text, instructions that explicitly present material rather than requiring subtle inferences (which requires more processing), would be better.

This theory that age-related cognitive decline is due to decreased processing resources also suggests that automatizing components of complex behaviors would be an effective strategy for older adults.

Any skill that is practiced sufficiently becomes "automatized" (think of driving a car or playing the piano). A skill or habit that has been practiced to sufficient level to become automatic will never be completely lost. Unfortunately, research does suggest that older adults require a lot more practice than younger adults to achieve automatization - but the benefit to them may well be greater.

Other theories for age-related cognitive decline

It has also been theorized that age-related cognitive decline may result primarily from the slowing down that occurs with age. There is certainly little doubt about the fact of age-related slowing. But it seems likely that there is more involved than simply this, as age differences are still found on many tasks even when there is unlimited time to do them. It may well be that there is an interaction between slower processing and decreased capacity, causing timing to be more critical in complex situations (e.g., approaching a complex traffic interchange on a freeway at relatively high speed). Practice does improve speed in older adults (though not to the level that it does in younger adults).

Another theory is that older adults develop problems with the inhibitory mechanisms in working memory (the part of our brain that enables us not to pay attention to irrelevancies), and it is this that gives the impression that there has been a decrease in processing resources. A faulty inhibitory mechanism would cause older adults to pay more attention to irrelevant detail and encourage incorrect interpretations of context. There is some evidence that older adults find irrelevant information more distracting than do young adults.

  1. Park, Denise C. Applied cognitive aging research. Pp449-93. In Craik, Fergus I. M. & Salthouse, Timothy A. (eds). 1992. The Handbook of Aging and Cognition. Hillsdale, NJ: LEA. Pp111-165.