As I read MK Streznewsky's book, I was also intrigued by this 20% of inmates who are gifted.

I ended up finding some communications which might interest you, as I think they are the source of information for MK Streznewsky.

Harvey, S., & Seeley, K. (1984). An investigation of the relationships among intellectual and creative abilities, extracurricular activities, achievement, and giftedness in a delinquent population. Gifted Child Quarterly, 28, 73-79.

The paper mentions :
This research was supported by the Office of Juvenile Justice Delinquency Prevention, National Institute of Juvenile Justice Delinquency Prevention, U.S. Department of Justice Grant No. 79JN-AX003

The population of this study was made up of youths who entered the Arapahoe County (Colorado) Juvenile Justice System.

approximately 700 youths became eligible for the study
288 of the 700 eligible youth were screened.
The remaining 412 refused to sign the permission slip or were otherwise not available.
The descriptive results from the sample studied indicate that the youth included in the study appeared to give responses to the tests administered which were within an expected range and thus were considered representative.
All the final analyses were done using the final 114 subjects who completed the full assessment.

tests used for the full assessment included
the Wechsler Intelligence Scale for Children-Revised (1974),
the Wechsler Adult Intelligence Scale (1955) for youth 17 years and older,
the Torrance Test of Creative Thinking (TTCT) (1966),
and the Wide Range Achievement Test (WRAT) (1978)

Approximately 18% or 48 of 268 of those screened turned out to be gifted in some way. Forty of these 48 youths actually scored in the top 3%, and an additional 26 youths achieved scores which would place them in the top 15% on the measures used.

more youth than would be expected statistically, appeared in this juvenile justice system. Further, several of these youth were just below the gifted cut-off and a sizable proportion scored in the extremely highest category of ability.

the gifted youths in this population differed from their normal counterparts in that they had very high abilities in the area of fluid intelligence and, in most cases, did less well on the achievement test in relation to these high fluid abilities.

Horn (1980) further describes fluid intelligence as being developed through incidental learning, and essentially an intelligence that is not taught or used in school.

The thinking style of these students is often characterized by a nonverbal quick perceptiveness.

Additional sources :

Seeley, K. (1984). Giftedness and delinquency in perspective. Journal for the Education of the Gifted, 8, 59-72.

Seeley, K. (2003). High risk gifted learners. In N. Colangelo & G. A. Davis (Eds.), Handbook of gifted education (3rd ed., pp. 444-451). Boston: Allyn & Bacon.