Working memory and intelligence are highly related constructs, but why?

Working memory and intelligence are highly related constructs, but why?
Roberto Colom et al., 2008

http://www.uam.es/personal_pdi/psicologia/fjabad/cv/articulos/intelligence/Workingmemoryandintelligence2008.pdf

Abstract:

Working memory and the general factor of intelligence (g) are highly related constructs. However, we still don't know why. Some models support the central role of simple short-term storage, whereas others appeal to executive functions like the control of attention. Nevertheless, the available empirical evidence does not suffice to get an answer, presumably because relevant measures are frequently considered in isolation. To overcome this problem, here we consider concurrently simple short-term storage, mental speed, updating, and the control of attention along with working memory and intelligence measures, across three separate studies. Several diverse measures are administered to a total of 661 participants. The findings are consistent with the view that simple short-term storage largely accounts for the relationship between working memory and intelligence. Mental speed, updating, and the control of attention are not consistently related to working memory, and they are not genuinely associated with intelligence once the short-term storage component is removed.

Working memory is (almost) perfectly predicted by g

Working memory is (almost) perfectly predicted by g
Roberto Colom et al., 2004 

http://jtoomim.org/brain-training/working%20memory%20is%20almost%20perfectly%20predicted%20by%20g.pdf

"The central factor in the human-abilities model is g, not fluid intelligence, spatial visualization, spatial relations, or perceptual speed, while the central factor in the theories of human information processing is WM (Jensen, 1998; Lohman, 2000). An adequate representation of psychometric g requires a number and variety of tests (Jensen & Weng, 1994) . The only two studies relating WM with g are the ones performed by Ackerman et al. (2002) and Sueb et al. (2002). The former study found a correlation of .70 between WM and g, while the later study found a correlation of .38 between spatial WM and g, and of .58 between verbal-quantitative WM and g. However, these two studies did not test the predictive power of g over WM and against other first-order latent factors. CFA models are informative with respect to the predictive power of g over WM. Considering the three reported studies in the present article, the loading of g over the WM latent factor averages .96. Therefore, WM is (almost) perfectly predicted by g (92% of explained variance)."

Working Memory:

>Working memory is the ability to explicitly form and maintain mental representations of information (e.g., a visual image, a word) and to manipulate these representations.<

David C. Geary; Male, Female; 2010

[Working memory and IQ are two sides of the same coin.]

Intelligence and Working Memory:

Central characteristics of highly intelligent people are their ability to represent more information in working memory than other people and their enhanced ability to consciously manipulate this information.

The Origin of Mind, David Geary, 2005