As children we all play with “Join the Dots” puzzle books. We join the dots to reveal a picture. Imagine instead a join the dots game with the stars. Trying to join up the billions of stars in the sky in a way that makes sense. This is a good analogy for the human brain. The brain consists of 86 billion neurons, the grey matter. Each neuron is connected to many thousands of others. There are estimated to be one hundred and sixty trillion connections. During our lifetime, but especially when we are young, we create those connections. “Joining the dots” allows us to make sense of our world. Those interconnections are insulated with white fatty myelin. There are one hundred thousand kilometres of white matter interconnections. Enough to travel around the earth two and a half times.
The Shrinking Brain
One of the few things that is certain about the ageing brain is that it is smaller. We are born with a brain that is a third of a litre. As we grow our brain grows with us. Our largest size occurs when we are forty. It will reach one point two litres. That average hides a huge variation across individuals. Remember that it is not necessarily the size of the brain that matters to intelligence. It is the size of the cortex and how the brain is organized that really count. The Neanderthals had a brain size bigger than modern humans. It is not our brain size that made us more successful than them, but the way our brain was structured.
The reason why evolution decrees that the brain shrinks beyond forty is not known. Despite this, early scan researchers associated the physical changes to the performance of the brain. For example, the shrinking of the hippocampus was associated with memory decline. This could have been driven by ageist expectations. Older people’s mental ability was expected to decline throughout their Third and Fourth Ages.
Other researchers tried to explain the effect of ageing on “intelligence” directly. Studying samples of different age groups they measured different cognitive abilities. They looked for patterns with age. The dominant theory that emerged was that with age, the speed of information processing in the brain slows. This was an elegant theory. It suggested a single common factor that affects all parts of the brain’s abilities.
There are inconsistencies in these results between studies of intelligence using different types of sample. Some used individuals from different age groups (cross-sectional studies). Others followed the same group of people over an extended period of years. These “longitudinal studies”, as they are called, are much more difficult to construct. They also took much longer for the results to be available. They show far less decline across many constructs. They also show declines that start much later in life. Many of the results found in cross-sectional studies do not seem to occur.
The Puget Sound Study
One of the most famous longitudinal studies of intelligence is in Seattle. Professor Warner Schaie started it in 1956 in an area called Puget Sound. Every seven years since then, the same tests were repeated with the original sample. He was testing the negative stereotype of age that “old” people lose their mental abilities and become “feeble”. He wanted to know whether there was a uniform decline in the different kinds of intelligence within individuals. He also wanted to know at what age those declines could be detected. Was there variations across individuals?
He was able to demonstrate the dangers with cross-sectional approaches. The longitudinal data demonstrates that at any given age, intelligence was improving with time. Each successive generation showed an improvement on the previous generation. This happened across all the measures he was using. Intelligence was still declining but at a slower rate. Like our bodies our brains are aging healthily. This was good news for us all. It was not such good news for researchers using cross-sectional samples.
A hypothetical cross-sectional study in (say) 1990 would need seventy-year-olds. Those people were born in the nineteen twenties. The twenty year olds in the sample would have been born in nineteen seventy. Those seventy year olds had less exposure to educational opportunities. They had different nutrition and grew up and aged in a completely different environment. There is an implicit assumption in cross-sectional analysis. In fifty years’ time the twenty-year olds are expected to look like the seventy-year-olds in the sample. The younger cohorts obviously had a very different start in life. The cross-sectional approach was overestimating the decline, because of the generational improvements.
We saw in Newsletter #025 that the separation between the brain and the body is a myth created by humans. We also know that people are healthier than they have ever been (#027). Fitness brings with it increased blood supply to the brain, improving its function. One reason at least for the generational improvements.
Prof. Schaie remained with the Seattle Study throughout his career. In 2005 he looked back over fifty years of work and the seven different data collections rounds. He tried to summarize his views.
The Seattle study measured five dimensions of intelligence with multiple tests of each. He concluded that there was no general decline in intelligence with age. There were no uniform patterns of decline as hypothesized by earlier researchers. “Processing speed” influencing all aspects of cognitive ability was not visible. There were variations across the different dimensions of intelligence. There were massive variations across individuals.
Perceptual Speed peaks in the twenties. Numeric Ability starts to decline in our mid-thirties and then declines slowly as we age. Apart from these two, the other dimensions do not decline until at least the age of eighty. A twenty-five-year-old when they are eighty, shows no decline in Verbal Ability. There is very limited decline in Inductive Reasoning, Verbal Memory and Spatial Orientation.
His conclusion is important and refutes many old age stereotypes. The ageist view of “feeble” Third Agers does not happen in his data. He looks at the practical implications of the declines he has found. He concludes:
“It is not until the 80’s are reached that the average older adult will fall below the middle range performance of young adults…”
He goes on to address the specific issues of employment:
Hence, it turns out that for decisions relating to the retention of individuals in the workforce, chronological age is not a useful criterion for groups and certainly not for individuals”.
He points out that these results are independent of any expertise gained over a life time of work. Such expertise can counteract losses in basic cognitive ability. Generalizing about “older people” lies at the heart of ageism. The idea that older people decline mentally in lockstep is clearly wrong. Prof. Schaie’s results can provide ample evidence to show that this is not the case.