<p>In general, the introduction section of McArdle et al (2002) provides a good literature review on the topic (see link to PDF in references).</p>

<h3>Early research by Jones and Conrad (1933)</h3>

<p>McArdle et al (2002) extract a quote from Jones and Conrad's (1933) summary of the literature based on large cross-sectional studies and also the classic Army alpha tests:</p>

<blockquote>
  <p>"The chief characteristic of the curve may be summarized as involving
  a linear growth to about 16 years, and a negative acceleration beyond
  16 to a peak between the ages of 18 and 21. A decline follows,
  which is much more gradual than the curve of growth, but which by
  the age of 55 involves a recession to the 14-year level. (p. 239)"</p>
</blockquote>

<h3>Cattell on Crystalised and Fluid Intelligence</h3>

<p>Also, see Figure 1 in McArdle et al (2002) which extracts a diagram from Cattell (1987) that shows Cattell's theorised relationship between age and two types of intelligence, <a href="http://en.wikipedia.org/wiki/Fluid_and_crystallized_intelligence" rel="noreferrer">crystalised and fluid</a>. The theory suggests that fluid intelligence increases at a faster rate and peaks at around 18 and then shows a steady decline, whereas crystalised intelligence increases throughout the life span, with the rate of learning decreasing over time, and arguably approaching a plateau around 50 before commencing a very gradual decline around 60.</p>

<h3>References</h3>

<ul>
<li>Cattell, R. B. (1987). Intelligence: Its structure, growth and action. Amsterdam: North-Holland. </li>
<li>Jones, H. E., &amp; Conrad, H. S. (1933). The growth and decline of intelli-
gence: A study of a homogeneous group between the ages of ten and
sixty. Genetic Psychology Monographs, 13(1), 223–275</li>
<li>McArdle, J., Ferrer-Caja, E., Hamagami, F., and Woodcock, R. (2002).
Comparative longitudinal structural analyses of the growth and decline
of multiple intellectual abilities over the life span. Developmental
Psychology, 38(1):115.
<a href="http://www.iapsych.com/wj3ewok/LinkedDocuments/McArdle2002.pdf" rel="noreferrer">FREE PDF</a></li>
</ul>

<p><em>NOTE: After posting the question, I did a little research and found the above research. I've self-awarded the answer for sake of closure, but I'm happy to award to someone else with a better answer.</em></p>

CC BY-SA 3.0
2012-02-06T03:21:55.120Z
2012-02-06T03:53:05.767Z
2012-02-06T03:53:05.767Z
Jeromy Anglim
<p>In the classic review of intelligence research (<em><a href="http://www.psych.uiuc.edu/~broberts/Neisser%20et%20al,%201996,%20intelligence.pdf" rel="nofollow">Intelligence: Knowns and Unknowns</a></em>), Neisser et al (1996) write the following</p>

<blockquote>
  <p><strong>Stability</strong>. Intelligence test scores are fairly stable during
  development. When Jones and Bayley (1941) tested a sample of children
  annually throughout childhood and adolescence, for example, scores
  obtained at age 18 were correlated r = .77 with scores that had been
  obtained at age 6 and r = .89 with scores from age 12. When scores
  were averaged across several successive tests to remove short-term
  fluctuations, the correlations were even higher. The mean for ages 17
  and 18 was correlated r = .86 with the mean for ages 5, 6, and 7, and
  r = .96 with the mean for ages 11, 12, and 13. (For comparable
  findings in a more recent study, see Moffitt, Caspi, Harkness, &amp;
  Silva, 1993.) Nevertheless, IQ scores do change over time. In the same
  study (Jones &amp; Bayley, 1941), the average change between age 12 and
  age 17 was 7.1 IQ points; some individuals changed as much as 18
  points.</p>
</blockquote>

<p>Typically, IQ scores are standardised for age and thus remove age related differences.</p>

<p>My question is</p>

<p><strong>What is the  relationship between age and raw intelligence test score at the group-level from around age 5 and into adultyhood?</strong></p>

<p>From what I've read, I understand that at the group-level intelligence increases monotonically with age during childhood and that it levels-off at some point (perhaps around 18?). Also, below a certain age (e.g., in infancy), completely different tests are required to measured intelligence. I also understand that individuals may experience various trajectories (e.g., some might plateau earlier, etc.), but I'm interested in the group-level. Thus, an ideal answer would probably provide a sense of the functional form of the increase in intelligence over childhood and adolescence. </p>

<p>My first thought for an answer was to look at the test manual for a standardised child intelligence test, such as the <a href="http://en.wikipedia.org/wiki/Wechsler_Intelligence_Scale_for_Children" rel="nofollow">WISC</a>, but I don't currently have easy access to that.</p>

<h3>References</h3>

<ul>
<li>Jones, H. E., and Bayley, N. (1941). The Berkeley Growth Study. Child
Development, 12, 167-173.</li>
<li>Moffitt, T. E., Gabrielli, W. F., Mednick, S. A., &amp; Schulsinger, E (1981).
Socioeconomic status, IQ, and delinquency. Journal of Abnormal
Psychology, 90, 152-156.</li>
<li>Neisser, U., Boodoo, G., Bouchard, T. J., Boykin, A. W., Brody, N.,
Ceci, S. J., Halpern, D. F., Loehlin, J. C., Perloff, R., Sternberg, R.
J., and Urbina, S. (1996). Intelligence: Knowns and unknowns. American
Psychologist, 51:77-101. <a href="http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf" rel="nofollow">FREE PDF</a></li>
</ul>

<intelligence><developmental-psychology><aging><iq>
How does raw IQ change over the lifespan at the group-level?
Jeromy Anglim
# How does raw IQ change over the lifespan at the group-level?

(Asked by: username_0 on 2012-02-01)

In the classic review of intelligence research (*[Intelligence: Knowns and Unknowns](http://www.psych.uiuc.edu/~broberts/Neisser%20et%20al,%201996,%20intelligence.pdf)*), Neisser et al (1996) write the following

> **Stability**. Intelligence test scores are fairly stable during
> development. When Jones and Bayley (1941) tested a sample of children
> annually throughout childhood and adolescence, for example, scores
> obtained at age 18 were correlated r = .77 with scores that had been
> obtained at age 6 and r = .89 with scores from age 12. When scores
> were averaged across several successive tests to remove short-term
> fluctuations, the correlations were even higher. The mean for ages 17
> and 18 was correlated r = .86 with the mean for ages 5, 6, and 7, and
> r = .96 with the mean for ages 11, 12, and 13. (For comparable
> findings in a more recent study, see Moffitt, Caspi, Harkness, &
> Silva, 1993.) Nevertheless, IQ scores do change over time. In the same
> study (Jones & Bayley, 1941), the average change between age 12 and
> age 17 was 7.1 IQ points; some individuals changed as much as 18
> points.

Typically, IQ scores are standardised for age and thus remove age related differences.

My question is

**What is the relationship between age and raw intelligence test score at the group-level from around age 5 and into adultyhood?**

From what I've read, I understand that at the group-level intelligence increases monotonically with age during childhood and that it levels-off at some point (perhaps around 18?). Also, below a certain age (e.g., in infancy), completely different tests are required to measured intelligence. I also understand that individuals may experience various trajectories (e.g., some might plateau earlier, etc.), but I'm interested in the group-level. Thus, an ideal answer would probably provide a sense of the functional form of the increase in intelligence over childhood and adolescence.

My first thought for an answer was to look at the test manual for a standardised child intelligence test, such as the [WISC](http://en.wikipedia.org/wiki/Wechsler_Intelligence_Scale_for_Children), but I don't currently have easy access to that.

### References

* Jones, H. E., and Bayley, N. (1941). The Berkeley Growth Study. Child
  Development, 12, 167-173.
* Moffitt, T. E., Gabrielli, W. F., Mednick, S. A., & Schulsinger, E (1981).
  Socioeconomic status, IQ, and delinquency. Journal of Abnormal
  Psychology, 90, 152-156.
* Neisser, U., Boodoo, G., Bouchard, T. J., Boykin, A. W., Brody, N.,
  Ceci, S. J., Halpern, D. F., Loehlin, J. C., Perloff, R., Sternberg, R.
  J., and Urbina, S. (1996). Intelligence: Knowns and unknowns. American
  Psychologist, 51:77-101. [FREE PDF](http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf)

**Tags:** intelligence, developmental-psychology, aging, iq


---

## 1 answers

### Answer by username_0 on 2012-02-06. Score: 6 | Accepted answer

In general, the introduction section of McArdle et al (2002) provides a good literature review on the topic (see link to PDF in references).

### Early research by Jones and Conrad (1933)

McArdle et al (2002) extract a quote from Jones and Conrad's (1933) summary of the literature based on large cross-sectional studies and also the classic Army alpha tests:

> "The chief characteristic of the curve may be summarized as involving
> a linear growth to about 16 years, and a negative acceleration beyond
> 16 to a peak between the ages of 18 and 21. A decline follows,
> which is much more gradual than the curve of growth, but which by
> the age of 55 involves a recession to the 14-year level. (p. 239)"

### Cattell on Crystalised and Fluid Intelligence

Also, see Figure 1 in McArdle et al (2002) which extracts a diagram from Cattell (1987) that shows Cattell's theorised relationship between age and two types of intelligence, [crystalised and fluid](http://en.wikipedia.org/wiki/Fluid_and_crystallized_intelligence). The theory suggests that fluid intelligence increases at a faster rate and peaks at around 18 and then shows a steady decline, whereas crystalised intelligence increases throughout the life span, with the rate of learning decreasing over time, and arguably approaching a plateau around 50 before commencing a very gradual decline around 60.

### References

* Cattell, R. B. (1987). Intelligence: Its structure, growth and action. Amsterdam: North-Holland.
* Jones, H. E., & Conrad, H. S. (1933). The growth and decline of intelli-
  gence: A study of a homogeneous group between the ages of ten and
  sixty. Genetic Psychology Monographs, 13(1), 223–275
* McArdle, J., Ferrer-Caja, E., Hamagami, F., and Woodcock, R. (2002).
  Comparative longitudinal structural analyses of the growth and decline
  of multiple intellectual abilities over the life span. Developmental
  Psychology, 38(1):115.
  [FREE PDF](http://www.iapsych.com/wj3ewok/LinkedDocuments/McArdle2002.pdf)

*NOTE: After posting the question, I did a little research and found the above research. I've self-awarded the answer for sake of closure, but I'm happy to award to someone else with a better answer.*


---

