Species variability as individual differences

We have seen how Wallace and Darwin's theory of evolution requires a source of variation to make it function as it does. We also know that sexual reproduction and mutations in DNA sequences are the sources of this variability. As a result, each organism --with exceptions of those arising from the same egg-sperm combination (e.g. identical twins) -- has many differences. These individual differences appear at all levels -- including biochemistry, cognitive abilities, appearance, and personality.

Individual differences in ability and behavior

In previous chapters, Glietman has been concerned with general principles that more or less apply to all people. Now he turns to ways in which humans characteristically differ. Psychologists have traditionally studied these differences under the heading of personality and in regard to cognitive or intellectual abilities, "intelligence."

Assessing these individual differences

From the earliest days of scientific psychology in the late 1800s, there was an effort to develop tools to measure these differences and relate them to various needs. These included achievement, aptitude for certain occupations, ability to succeed in school, extent of certain disabilities, and other individual characteristics perhaps predisposing an individual to success, failure, disease in a wide variety of possibilities.

Measuring differences -- "the tools"

Variability and its measurement

Frequency distributions (frequency histograms)

Early statisticians began collecting data, displaying how often certain categories of some dimension occurred relative to others. These displays are called, naturally enough, frequency distributions.

The "normal" distribution or "normal curve"

Many measured characteristics were found to have a frequency distribution that resembled a bell-shaped curve. These distributions can be modeled mathematically by the famous "normal distribution" where frequency, Y, is a function of the proportion of cases with a certain value, X.

Two important features of such distribution are:

(1) the measure of central tendency, e.g. the average value

(2) the variability the measure, e.g. its variance, that reflects on average how much individual scores tend to differ from each other. For example if they are all the same there is no variance.

Relationship beween variables -- scatter diagrams and correlation coefficients

Correlation and regression were devised by Francis Galton (1822-1911) as a means of evaluating the extent a trait might be hereditary, e.g. correlating heights of sons and their fathers vs their teachers. Once differences were measured, questions arose about the relationships among them. Francis Galton (1822-1911) devised the correlation coefficient, r, as an index of predictability or association. He wanted to measure, for example, the degree children were like their parents. They could be exactly like them, r=1.0 or they could be totally independent -- knowing the parents' height tells us nothing about the child's height, r=0. (This was in the context of Wallace and Darwin's theory of evolution -- remember for that to work advantageous traits had to be inherited to some extent by offspring.) (See Gleitman for discussion of correlation.)

The method is still used; it is particularly informative in twin studies, comparing identical with fraternal twins on some dimension.

"factors" - sets of correlated variables

The idea of factors is both mathematical and psychological. It is one method of determining traits from behavior. See Gleitman's sea serpent analogy (641). In connection with individual differences, especially intelligence ("g") but also personality ("big five"), it is an attempt to reduce superficial differences to more fundamental psychological differences.


This is a term referring to the extent to which variability of something in a particular population --not an individual -- is due to genetic variability as opposed to other, environmental factors. It is sometimes expressed as a proportion or percentage of that variance. (It's often tricky to measure in practice; see G on PKU, p.661 and Fig.15.17) Even a high degree of heritability does not imply a trait cannot be affected by changes in environment. This especially applies to controversial concepts like "intelligence." See the news item on the new paper by Dickens and Flynn (2001).

(Heritabiltiy is an extension of Galton's work on measuring inherited traits.)

The idea of a test

Test development -- finding problems that are reliable and valid indicators of some trait or ability


Predictive validity (evidence a test is valid because performance correlates with an external criterion, e.g. SAT scores and college grades)

construct validity (test performance fits into theoretical schema)

Reliability -- the consistency of a test

This is usually measured using a correlation coefficient -- perfect reliability is 1.0.

Standardization -- developing norms of performance

Comparison of individuals by percentiles, standard scores

This requires a representative sample of scores against which to compare any individual's score. Often a normal distribution is assumed.

Test administration -- the manual and training

Giving most tests require extensive training and familiarity with the test manuals. The manuals describe the history of the test, data on reliability and validity, the standardization procedures, and provide information on administering and scoring each test item.

Finding an appropriate test

There are publications that review available tests and the APA maintains a webpage on the topic of finding tests. Developing and selling tests is a big business. While the Stanford-Binet has long been used for children, the WAISis now widely used with adults. New tests are always being developed and evaluated. See, for example, the Buros Yearbook of Mental Measurement or visit the reference area in the library.

While you may never have need of a test, you might want to learn more about a test or measure used on yourself or family.

Personality -- differences that presumably matter in behavior

various personality characteristics or traits

(These differences will be discussed later; see Gleitman, ch. 16) The point here is just that most of the same methodology developed for measuring intelligence has been applied to other differences as well.)

Mental differences -- several conceptions

Psychometric "IQ"

Mental age, chronological age, standardized scores

Biological differences?

Brain size (relative)

Brain complexity (e.g. more synapses per neuron)

other suggestions include more glial cells (Einstein's brain has more than average), better myelination (enhances speed of processing), and the many effects of nutrition (See below.)

Information processing differences?

Intelligence is a product of interactive use of memory, reasoning, attention and other cognitive processes including speed of processing.

Single factor "g" vs multiple factors in intelligence?

There is a long-standing debate on this -- sometimes based on subtle arguments about the strength and size of sets of correlation coefficients and the use of factor analysis. See Gleitman's sea serpent analogy. In clear cases, one could say a given set of items correlate among themselves but not with other items. Each set independent set of items would be a factor. For example, people who do well on vocabulary items do not necessarily do well on block design or box-unfolding tasks. Unfortunately in practice, instead of correlations like .07 or .94, one often finds .43 and .52.

Environmental factors and intelligence

The Flynn effect

Absolute scores on intelligence tests have been increasing steadily since tests were widely used. The reason for this is not clear but nutritional differences and education in the use of "mind-tools" are plausible explanations. See the recent news item on this.

Many known factors influence performance

One example involves the role of nutrition and how it affects many aspects of mental development. (See Limber's diagram below.)

Ideas on "Intelligence" -- the good, the bad, the ugly

Here are some representative ideas; it is clear why this is a controversial topic, especially when linked to heredity. (Keep in mind though that it is only since the turn of the century that heredity was recognized as a relatively unchangeable entity. Darwin and most psychologists educated before 1920 including Freud, believed in Lamarckism to some extent. This meant that children's intelligence --even if were totally inherited -- would improve if their parent's abilities were improved by schooling, training, etc. Of course this is exactly what the Flynn effect has revealed; of course Lamarckism is false but the education does appear to have the effect the Lamarckians believed in -- just for the wrong reasons.)


Descartes (1596-1650) responding to Montaigne's "Essays" on abilities of animals, argues that automatic or instinctive ability in animals does not show reason; instead it shows none!

"nature makes them (animals) behave as they do according to the disposition of their organs; just as a clock, composed only of wheels and weights and springs can count the hours and measure the time more accurately than we can with all our intelligence. Descartes, R. (1641) Discourse on Methods, Fifth Part.

A modern example, not so harsh on the animals, is to compare a human child's acquisition of language with efforts to teach nonhumans (chimps, dogs, parrots) to use a human-like language. We are much better at it -- no kidding -- but not because we are more intelligent, but because it is largely innate in us. But the poor ape or dog has to struggle using whatever intelligence it has to master our language. It is no surprise then that other than for vocabulary size, there is little relation between human language acquisition and IQ scores.

The bottom line here is that it is how behavior comes about that is relevant to judgements of "intelligence" -- not just ability to perform.


Spencer (1820-1903) was a very original thinker, with many ideas -- including some bad ones -- on evolution is sometimes credited with founding sociology.

...well developed nervous systems will display a relatively marked premeditation--an habitual representation of more various possibilities of cause, and conduct, and consequence--a greater tendency to suspend judgments and an easier modification of judgments that have been formed. Those having nervous systems less developed, and with fewer and simpler sets of connexions among their plexuses, will show less of hesitation--will be prone to premature conclusions difficult to change. Unlikenesses of this kind appear when we contrast the larger brained races with the smaller brained races--when from the comparatively judicial intellect of the civilized man, we pass to the intellect of the uncivilized man, sudden in inferences , incapable of balancing evidence, and adhering obstinately to first impressions. And we may observe a difference similar in kind but smaller in degree between the modes of thought of men and women; for the women are the more quick to draw conclusions, and retain more pertinaciously the beliefs once formed......



"My object in this chapter is soley to shew that there is no fundamental difference between man and the higher animals in their mental faculties." 35 ..I have found….(among) animals of many kinds, including birds, that the individuals differ greatly in every mental characteristic..36"

"No doubt that it is always difficult to distinguish between the power of reason and that of instinct.. 46"

(from Descent of Man, 1871) Darwin knew that domestic animals had smaller brains than their wild counterparts and that "smart" ants had larger nervous systems than "duller" ants -- hence size of the nervious system somehow mattered.


Galton (1822-1911) was Darwin's cousin and a prominent scientist, who among other accomplishments invented the concept of linear regression in his effort to understand the heredity of traits as well as actually try to quantify intelligence. He was also a founder of the widespread eugenics movement to improve the human race. By 1931, thirty states in the US had passed sterilization laws aimed at "improving the race." Today this development is seen as a terrible mistake.

"I propose to show in this book that a man's natural abilities are derived by inheritance, under exactly the same limitations as are the form and physical features of the whole organic world. Consequently, as it is easy, notwithstanding those limitations, to obtain by careful selection a permanent breed of dogs or horses gifted with peculiar powers of running, or doing anything else, so it would be quite practicable to produce a highly gifted race of men by judicious marriages during several consecutive marriages."

Galton, F. (1869). Hereditary genius, an inquiry into its laws and consequences .


Binet was a member of a group dedicated to improving education and developmental opportunities. In 1904 the French Minister of Public Instruction appointed a commission to develop objective criteria for the educability of children. Binet and Simon essentially used teachers judgments about necessary skills and objectified them in their tests. Hence Binet's test and those derived from it inherently have "intelligence" related to success in formal schooling -- the use of "mind-tools."


Piaget considers intelligence to be governing adaptive processes for all members of a species. Although he worked in Binet's lab, he was not interested in individual differences but in developmental differences that were uniformly dependent on children's developmental experiences.


Terman -- a psychologist at Stanford University --adapted Binet's test (1916) and added a number of features especially standardization and applied it across the board expanding its use from determining mental retardation to scaling "intellect" for many purposes including identification of high ability individuals. He began a long term study of individual's with high scores on his test, known today as the Stanford-Binet.


Working with animals, Harlow developed the idea that intelligence was the ability to "learn to learn" arbitrary problems -- an indication of adaptive capability.

Gardiner's "multiple intelligences"

(See Gleitman and video. While he has identified a number of abilities that are important for success in modern society, should all these be called intelligences?)

Skill at using "mind-tools" like language, math

One possible cause of the Flynn effect, along with nutrition?

Inheritance or environmental factors-- a misleading dichotomy?

See the example of nutrition and mental development discussed in class; as the complexity of an individual's performance becomes better understood, simple ideas of nature and nurture become less appealing. As Glietman's discussion of the PKU problem indicates, in some environments, a given set of genes can be very detrimental yet in another, not notable at all. This should not surprise us as it is inherent in the idea that "fitness" is relative to specific environments.

tests as excluders or gatekeepers

Supporters of tests can usually point out some useful functions attained with their use. However predictive validity is rarely perfect and one consequence is that individuals who might be capable of such and such are not allowed the opportunity because a moderately valid test has been used for selection. In recent years it has become clear many students do well in college who didn't do so well on SAT type tests. Even worse are tales of companies using IQ tests as measures of ability in specific jobs, e.g. gardener, drill press operator, salesperson where little validity is present. APA standards and legal precedents have increasingly ruled against the use of tests without validity and/or with high error rates.

An "IQ" gene?

Recent findings showing that a single gene affected a rat's ability to find its way through a maze was hyped in Time Magazine and other media as a major discovery. Was it? How does it impact our thinking about "intelligence." It is a long way from a gene that facilitates forming and loosening associations in rat cognitive maps to the human concept of intelligence. Surely many of our 30,000 genes work in concert to enable our many abilities.


(revised, 4/16/01)