One Number Could Not Hold Them

What the research found

The Wechsler Intelligence Scale for Children, now in its fifth edition (the WISC-V), is one of the most widely used intelligence tests for children in the United States. Schools often lean on a single number from it, the Full Scale IQ, to decide who enters a gifted programme. A new study asks a simple question: for gifted and twice-exceptional children, does that single number actually mean anything?

The researchers, led by Barbara J. Gilman and colleagues, examined the WISC-V records of 390 children referred by their parents for testing across seven sites in the United States. The children were gifted, highly gifted, or twice-exceptional, meaning gifted while also having a disability such as dyslexia, ADHD, or an autism-spectrum profile. The Full Scale IQ is built by combining scores from several separate areas, called indexes, that measure things like verbal reasoning, visual-spatial reasoning, working memory, and processing speed. The test’s own manual warns that when those areas are too far apart, the combined score should not be treated as a single, meaningful figure.

That warning turned out to matter a great deal. In this sample, 286 of the 390 children, roughly 73 per cent, showed a gap of 23 points or more between at least two of their index scores. A 23-point gap is about one and a half standard deviations, the size of difference at which the test manual itself says the Full Scale IQ becomes uninterpretable. By that rule, the summary score was a meaningful measure of ability for only about 30 per cent of these children. For the rest, the number that schools rely on was, technically, not something the test was designed to produce.

The pattern behind the gaps was consistent. These children tended to score highest on the indexes that draw most heavily on abstract reasoning, and lower on processing speed, which measures how quickly a child completes simple timed tasks. Processing speed has little to do with the reasoning ability that defines giftedness, yet it is folded into the Full Scale IQ and can pull the total down. When a bright, uneven profile is averaged into one figure, the averaging can hide the very strength the test was meant to detect.

The authors then looked at alternative scores designed to sidestep this problem. The General Ability Index, or GAI, sets aside working memory and processing speed and focuses on reasoning. In this group the mean Full Scale IQ was about 127, while the mean GAI was about 129, and the GAI documented high ability in roughly 85 per cent of the children, compared with 30 per cent for the Full Scale IQ. A broader version, the Expanded General Ability Index, draws on eight reasoning subtests across the verbal, visual-spatial, and fluid-reasoning areas. These strength-based indexes, the authors argue, capture what the Full Scale IQ misses in this population.

What it means for you and your child

If your child has been tested on the WISC-V, the most useful thing to take from this study is a question to ask, not a verdict to accept. Ask whether the Full Scale IQ was interpretable in the first place. If the gaps between your child’s index scores are large, that single number may be, in the test’s own terms, not a valid summary of their ability. A score that looks disappointing on paper can be an artefact of averaging a spiky profile, rather than a true ceiling on what your child can do.

This matters most for twice-exceptional children, whose profiles are often the most uneven of all. A child can have exceptional reasoning and slow processing speed at the same time; a disability can suppress one index while giftedness lifts another. When those are blended into one figure, both the giftedness and the disability can disappear from view. Asking for the separate index scores, and for reasoning-based measures such as the GAI or the Expanded GAI, can make both halves of the picture visible again, which in turn can matter for admission to programmes and for getting support for a disability.

It is worth being clear about what this study does and does not establish. It is a single study of 390 children who were referred for testing, mostly by parents who already suspected giftedness. That is not a random sample of all children, so the exact percentages here should not be read as fixed rates for the wider population. The study shows that discrepant profiles are common in this group and that the Full Scale IQ frequently fails its own interpretability standard for them. It does not prove that any one child’s true ability is a particular number, and it cannot tell you, on its own, which programme your child belongs in.

Nor does it show that the alternative indexes are flawless. The General Ability Index gives a fuller reading of reasoning strength precisely because it leaves out working memory and processing speed, but those skills are still real and can shape how a child manages schoolwork day to day. A high reasoning score does not erase the practical effect of slow processing or a specific learning difference. The right response is not to swap one magic number for another, but to read the whole profile, strengths and weaknesses side by side.

The steady conclusion is an old one, now supported by fresh data. No single score can describe an uneven mind. For a gifted or twice-exceptional child, the interesting information often lives in the gaps between the scores, not in the average that smooths them away. If a decision about your child is resting on one figure, it is reasonable, and now well grounded, to ask what that figure is hiding.