Intelligence research grew as the study of comparatively-tested groups - and indeed the IQ measure is comparative, since it describes relative performance on these tests compared with a reference group.
But this is a very strange and unusual way for a biological subject to originate - since most biology developed on the basis of individual studies - with groups merely being used to check that individual observations were not unique, and to increase the precision of quantitative estimates by the use of averaging.
So IQ research has suffered this distortion - and the IQ measure has seldom been very useful for understanding individuals - except when there is a dissociation between general intelligence and other abilities.
(In contrast, IQ is very useful for predicting group level performance.)
Furthermore, this distortion has gone to the very heart of intelligence research, and has - I judge - exerted a paralysing effect on it; since the conceptual understanding of what-intelligence-actually-is remains pathetically weak.
The simple reason for the feebleness of understanding of the central concept of intelligence research (by its leading researchers!) - is that IQ, and indeed general intelligence/ g itself - is a group-level comparative measure
- but what is needed to understand intelligence a description of what happens at the individual, personal level - without reference to other people.
In sum, intelligence research needs individual case studies - it needs very rich and detailed descriptive studies of the cognitive functioning of specific people in order to be able to develop an understanding of what-intelligence-is.
Or, to put matters another way, until intelligence researchers can do conceptualize and conduct proper (scientifically valid) individual case studies, then they won't really know what they are talking about.
(More to follow on this theme...)
A critique of biological psychiatry. Psychological Medicine, 1990;20:3-6
Endocrine physiology and the value of case studies. Journal of Endocrinology, 1991;130:1-2
The scope and nature of epidemiology. Journal of Clinical Epidemiology, 1996; 49: 623-626.
Charlton BG, Walston F. Individual case studies in clinical research. Journal of Evaluation in Clinical Practice 1998; 4: 147-155.