by Ian J. Deary
Schaie added an interesting twist, making the study much more useful and, especially as time went on, more burdensome to organize. Look at the second grey column of Figure 8. In 1963, when the people from the 1956 sample were coming back to be tested for a second time, Schaie recruited a new group of participants, to be tested for the first time. The second grey column of Figure 8 shows that he collected data on 997 completely new subjects. These people, again, were aged from their late teens to their 80s, and were given the same tests every 7 years from then on. Schaie’s approach is now obvious. He collected a completely new group of several hundred people (aged from late teens to 80-something) every 7 years and asked back and tested all the old groups as well. Every 7 years, then, each of the groups already recruited was asked back to be tested again, and a new sample was collected.
All this means three things. One: the bottom of every one of the grey columns in Figure 8 marks a new cross-sectional study of ageing and intelligence. This informs us about the age-related differences in mental abilities in samples taken from different decades of the second half of the 20th century. Two: each column of Figure 8 is a new longitudinal study, which allows some judgement to be made about whether the results from any one longitudinal study are able to be repeated. Three: most crucially, this type of study allows one thing that neither a cross-sectional or a longitudinal study alone can do. We can compare people of the same ages at different years in history. Thus, Schaie’s design allows us to ask the question of whether, say, 20-year-olds (or 30-, 40-, or 50-, and so forth) in 1956 score the same as 20-year-olds (or whatever) in 1963, 1970, 1977, 1984, and so on. This key question is called a ‘cohort’ effect, and is looked at in more detail in Key dataset 10 (Chapter 6).
9. Not all aspects of intelligence show the same patterns of ageing. Examples of two test results from K. Werner Schaie’s Seattle Longitudinal Study. Inductive reasoning – working out general rules from specific examples – declines with age from some point in the 30s. Verbal ability shows no appreciable decline with age.
As you will appreciate, it is not easy to summarize the Seattle study, because of the large amount of data it produced and the fact that it has been reporting results for over 30 years. However, some aspects may be summarized briefly. The ‘cohort’ effects do exist, with later generations scoring better than their predecessors at the same age (see Key dataset 10 in Chapter 6). The longitudinal aspects of the study do show practice effects on the tests. The cross-sectional data show a fairly straight decline from age 25 to 80 years in inductive reasoning (discovering a rule from a limited number of instances), spatial orientation (making decisions about complex shapes in two or three dimensions), perceptual speed (the ability to notice fine visual details quickly), and verbal memory. There was a peak in middle age, and much less age-related decline in verbal and numerical ability. Figure 9 shows some quite typical results from Schaie’s study. Verbal ability peaks in the 30s and stays stable until old age. Inductive, abstract reasoning declines from young adulthood to old age.
It is possible to put together all of the studies we have discussed so far concerning the ageing of intelligence and make some general conclusions. There are common characteristics of those tests that are stable over age and those that show decrements. Tests on which we can all hope to be performing well in our old age are those that involve knowledge or educational experience and draw generally from our stores of knowledge. These are called ‘crystallized’ abilities by psychologists, and the metaphor is used to indicate that we have formed the knowledge solidly in our brains. A good example of such a test is vocabulary.
Tests on which people beyond their 30s are typically already past their peak are those that involve more on-the-spot thinking, with novel material, and often completed under pressure of time. These are called ‘fluid’ abilities, indicating that they represent the current state of our brainpower. The distinction between fluid and crystallized intelligence was noted by John Horn and Raymond Cattell in the 1960s. A good example of a fluid ability test is Raven’s Progressive Matrices, in which one must find the correct example to finish an abstract pattern (an example was given in Figure 2). Thus, as a broad generalization, the tests which show decrements with ageing are those that involve speedy, active brainpower with ideas that we have never seen before. The tests that hold better with ageing are those that call upon our stored knowledge retrieved at our leisure. You might think of this distinction as ways of enquiring about the output of a factory. This could be done in two ways. You could go to the shop floor and ask to see the on-the-spot manufacture of some new products. This would tell you about the factory’s current ability to make new objects, about the present capability and current efficiency of its machinery: fluid intelligence. On the other hand, you could ask to be taken to the warehouse to see the quality and quantity of the products it has accumulated over its active lifespan: crystallized intelligence. On this type of thinking, Paul Baltes, an eminent researcher on the ageing of human mental abilities, made the following distinction. He called our present mental capabilities the ‘mechanics’ of our intelligence and our stored knowledge the ‘pragmatics’ of our intelligence. His decades of research in Germany, with the Berlin Aging Study and others, shows that old age reduces the mechanics, but the pragmatics hold up well as we grow old.
Thus, if we try to answer the question ‘does intelligence decline with age?’ we must answer in both the affirmative and the negative simultaneously, depending on the type of ability that is being discussed.
Let’s go back to Schaie’s Seattle study and discuss some more of their data. Apart from merely asking what types of ability do and do not change as we age, he noticed and wondered why some people seemed to preserve their thinking skills better than others as they grew older. This is a well-known phenomenon, but often overlooked. We tend to discuss the young and the old as if they were just one mass of each, with no individual differences. What Schaie’s study and others find is that there are large differences in mental ability changes with age: some people decline, some stay the same, and some even improve. Perhaps there is more human interest in this one question than most others: what are the factors that will help us to retain our mental abilities as we grow older? Can we buck the general ageing trend of our peers? What, then, predicts favourable cognitive ageing? Schaie found that the following factors contributed to holding on to one’s mental abilities:
– having no cardiovascular or other chronic disease
– living in a favourable environment mediated by high social class
– being involved in a complex and intellectually stimulating environment
– possessing a flexible personality style in midlife
– living with a spouse with high mental ability
– maintaining a fast level of processing speed in the brain
– being satisfied with life in middle age
Key dataset 5
The question I shall address now is: what exactly is it that declines when we say that mental ability declines with age? Let’s address those mental abilities that do show some decline with age: there are many of them. If we look through all of the research reports we can show that hundreds of types of individual mental test scores decline as we grow older. However, the first two Key datasets in Chapter 1 gave us a way to think about ageing and mental abilities. We can ask whether it is mostly the stratum III general factor that changes as we grow older, and/or particular stratum II group factors, like memory, spatial ability, processing speed, verbal reasoning, and/or specific abilities that live on stratum I. Therefore, psychologists are faced, potentially, with having to come up with an account of how many different abilities age, and providing a mechanism for each.
The leviathan in this field of research is Timothy Salthouse and Figure 10 captures his ideas, though the diagram is taken from one of the reports from the Berlin Study of Aging (details at the end). Let me give the punchline first and work backwards from it to the supporting data. Salthouse believe
s, after examining many data over more than three decades, first that age affects the general factor in mental ability and nothing much else. The fact that very specific mental abilities (stratum I in Chapter 1) or group factors (stratum II) show age-related changes, says Salthouse, is mostly because they relate to the general intelligence factor. Second, he believes, after examining much more data, that the decline of the general factor with age is mostly caused by a slowing of speed of mental processing.
10. A drawing to illustrate that the effect of age on specific mental skills acts via the effect of age on general mental ability. Some researchers think that the effect of age on general mental ability is due to a slowing of the brain’s processing speed.
Figure 10 will be our guide to this research area. Have a look at it. Note that I have put speed in dotted lines. That’s because it might be best placed in one of two different locations in the diagram. At the bottom of the Figure are several lines sticking out from 5 ellipses (familiar from Figures 1 and 4). They are types of mental ability (stratum II or group factors) and the lines sticking out are some different, individual tests that can be used to test them. I have given the stratum II mental abilities these specific names because they occurred in one particular research paper; however, the results are similar across most studies even when they examined other mental domains. Note that reasoning, memory, fluency, knowledge, and speed all have lines pointing to them from g (stratum III or general ability). This illustrates what we saw earlier, that almost all types of ability have positive relations with each other: people who are good at one tend to be good at all the others. Salthouse then asked an interesting question.
When we look at all those mental abilities that change with age, what does age affect? That is, does age affect general ability or does it have discrete effects on individual abilities? Perhaps, for example, it affects memory more than fluency, or reasoning more than knowledge, or perhaps speed in particular? These questions can be tested, but the statistical armoury that tests them is beyond my wit to explain here. However, the idea of the thing can be got across. Have a look at Figure 10 again. What Salthouse did was to assume that age affected only the general mental ability g. He was then able to ask whether that accounted for all the effects of age on the more specific mental ability test scores, or whether there were still substantial age effects that leaked over to the group factors and individual tests. The answer was clear: the effects of age were almost entirely and only on general ability. Once that was taken into account, there was almost no effect of age on the more narrow mental capabilities.
This very simple idea worked for many datasets that Salthouse analysed, including banks of other people’s data. And others, such as researchers in the large Berlin Study of Aging, found the same results (it is one of their diagrams that I used as a model for this Figure). So, what the Figure tells us is that age alters g (general ability) and that it is this change in general ability that affects all the different mental abilities we recognize. The reason that ‘reasoning’, ‘memory’, ‘fluency’, and ‘knowledge’ changed with age was because they were related to general ability; it was general ability that aged, not something special about any one of these group/special ability factors. Note that I refer to ‘change’ rather than decline. Although it is the case that on average these abilities will go down with age, some people within the groups stay the same or even get a bit better.
What does this result mean? It means that what ages when we talk of intelligence ageing is something very general – some broad capability of the brain to handle ideas is changing, not just specific aspects of mental function. Salthouse then asked why this should be. It is not enough to say that growing older causes mental changes, especially in general mental ability. We have to try to be more specific: we must think about what physically changes in the brain as we grow older to produce these effects. His guess was that all these abilities seem to change together because our ‘mental speed’ is slowing down as we get older. Therefore, his bold theory is that: (1) age causes slowing of mental speed (sometimes called speed of the processing of information); (2) this change in mental speed is the cause of the change in general ability; and (3) the change in general mental ability causes the change in many different, more specific abilities, like memory and so forth.
We need to have a word about how he and others in this field have measured mental speed. Sometimes they use mental tests that are part of intelligence test batteries. For example, a test called ‘digit symbol’ is sometimes used as a putative index of mental speed. It belongs in the Wechsler test collection we saw in Chapter 1 and is illustrated in Figure 3. The person has to write a symbol below a number according to a given code. Therefore, for each item in the test, the person looks at the number, looks over to the code, notices the little symbol that corresponds with the given number, and writes that symbol below the number. They do as many as they can in a given time. Older people tend to get fewer of these done than younger people. Sometimes researchers use more specialized tests that are only found in laboratories. For example, they might use tests of reaction time. This type of test measures how quickly a person can react to an event. It might involve pressing a button as soon as a light comes on, though it is usually more complicated than that. It might involve looking at a panel of four lights, waiting for one of them to be switched on, and pressing the correct button for that light as fast as possible. (There’s more about reaction times and how they are tested in Chapter 3.) Older people on average are slower at these sorts of tests. What’s special about these sorts of tests is that they are relatively simple. Generally speaking, people do not make any errors on these tests, especially if they are allowed to do them without time pressure. Therefore, whereas most mental tests, like memory and reasoning and so forth, can be difficult and lead to errors, these ‘mental speed’ tests are simple and look only at our rate of work when making very straightforward decisions. Researchers tend to use these tests as if they were telling us about some basic speed limitation of people’s brains in getting through mental operations.
If Salthouse’s idea is correct, the age effects on different mental abilities noted in a group of older people is caused largely by a change in general mental ability, and that change in general mental ability is due to changes in speed of information processing. Therefore, what seems like a kaleidoscope of mental change can to a great extent be explained by one simple fact: as we get older our rate of processing information in the brain slows down.
To an impressive extent Salthouse’s simple idea does work. He took many researchers’ data on mental abilities and age and tried out the same idea. He asked: once we remove the effects of mental speed, does age still affect general and specific mental abilities? The answer: hardly at all; when we take out the effects of mental speed on mental test scores we have removed most of the age effects too. To see what this means have another look at Figure 10. Here, we see Salthouse testing the idea that age itself does not directly affect general and specific mental abilities, even though we do know that things do change with age. Salthouse is stating that the effect of age is to slow down mental speed, that general ability declines when mental speed slows down, and that all the specific mental abilities then decline when general ability declines.
Psychologists in this type of area do try to be more specific about what they mean by mental speed. The tests they use to measure mental speed are certainly a bit more simple than the ordinary type of mental test, but they don’t really tell us what is happening in the brain. Things like ‘digit symbol’ and ‘reaction time’ tests are in fact still quite complex, because we do not understand how the brain performs these tasks or how their slowing translates into changes in the biology of the brain. There the story ends, I am afraid, as far as the science goes. At this point the researchers become rather metaphorical. The favourite metaphor is the computer. Most people who have bought a computer will have been told about various aspects of its performance. One of the principal parameters is the c
lock speed, the processing rate of the main processor. The faster it is, the faster the computer will work and the faster it will complete complex operations. Statistical analyses that took several hours in 1990 (I used to leave my computer running overnight) now take unmeasurably small fractions of a second. So, the metaphor runs, as we grow older our brain’s ‘main processor’ runs at a slower rate and we get the answers to mental problems more slowly, less accurately, or sometimes not at all. But a metaphor is no substitute for scientific explanation, and one necessary extension to these interesting findings is to realize the concept of ‘mental’ speed in terms of changes in the biology of the brain.
What research is currently going on in this area?
The study of cognitive ageing is arguably one of the most lively and exciting in the field of human intelligence – and arguably one of the most important, as the proportion of older people in the population grows larger and as people live longer and healthier lives. Indeed, about ten years ago the American Psychological Association started a new research journal called Psychology and Aging just to cope with the large amount of high-quality research that was taking place. Research interests are broadening in scope to ask questions such as: What are the causes of different rates of ageing of mental abilities? What are the mechanisms by which age impinges on mental abilities?
To follow this area up …
Here’s the paper that my research team working on the Scottish Mental Survey 1932 published on the follow-up mental testing 66 years later.
