Intelligence  is a complex phenomenon, reports leading researcher Ian Deary.  Everybody has a spectrum of mental abilities including language, memory, spatial ability, psychomotor speed, reasoning, etc.

When you test these abilities however, the results tend to correlate pretty strongly with each other. That is, people who are high scorers on one of them, will tend to be high scorers on all of them.

From the correlations amongst these abilities, a general mental ability factor (gma) is identified, and more often than not says Deary, it is the best predictor of things of practical significance such as academic achievement and occupational attainment.

While gma is important, it is not the whole story.        Two broad mental abilities are now identified that underpin our general mental ability: 

• Fluid ability - a person's natural ability which is independent of acquired knowledge. Also thought of as "raw" intelligence.
  
  
• Crystallized ability - a person's store of knowledge, information, skills and strategies accumulated over time.

All other mental abilities contribute to either our fluid ability or crystallized ability, and can be measured using specific tests.

How does cognitive ability affect your life?

Everybody knows life isn't just about a high score on a test. But cognitive ability can predict success in life as well as, if not better than any other measurable variable psychologists can test for.

Studies have shown that if you take a group of 18 year olds and follow them into their futures, people of higher intelligence have less chance of living below the poverty line and dropping out of high school and more chance of getting a college degree, no matter what their social class.

What influences intelligence?

This is an evolving area of research. There are always environmental and genetic influences that affect intelligence, and it looks like the genetic factors play more of a role as we age.

There are some interesting commonalities that link many intelligent people. To some extent intelligent people have:

• Larger brains as seen on MRI scans, and more efficient brain metabolism.
  
  
• Faster reaction times, better memory and more efficent sensory processing.
  
  
• Specific brain responses to stimuli when measured on an EEG  machine.
  
  

Does intelligence stay the same throughout your life?

Like so much of what is known about intelligence, the answer is yes, no and who knows? Crystallized ability seems to be fairly stable later in life, while fluid ability seems to fade. In other words, a person's knowledge and verbal skills hold up well as they age, while their ability to think under pressure with new materials is not as strong. A bright 25 year-old is likely to be a bright 65 year-old, but there is variation between individuals.

So far, research suggests the following factors could play a role in maintaining our mental abilities:

• Being free of chronic disease and getting plenty of intellectual stimulation.
  
  
• Having a flexible personality and being happy in the middle years.
  
  
• Having an intelligent partner.

Deary I.Differences in Mental Abilities. British Medical Journal 1998; 317:1701-1703