This is important work, but I have worried from time to time about the potential for misunderstanding. In particular, if you are a parent of a child with DLD, should you be alarmed at the prospect that your offspring will be incarcerated? So I wanted to give a brief explainer that offers some reassurance.
The simplest way to explain it is to think about gender. I've been delving into the latest national statistics for this post, and found that the UK prison population this year contained 82,314 men, but a mere 4,013 women. That's a staggering difference, but we don't conclude that because most criminals are men, therefore most men are criminals. This is because we have to take into account base rates: the proportion of the general population who are in prison. Another set of government statistics estimates the UK population as around 64.6 million, about half of whom are male, and 81% are adults. So a relatively small proportion of the adult population is in prison, and the numbers of non-criminal men vastly outnumber the number of criminal men.
I did similar sums for DLD, using data from Norbury et al (2016) to estimate a population prevalence of 7% in adult males, and plugging in that relatively high figure of 50% of prisoners with DLD. The figures look like this.
|Numbers (in thousands) assuming 7% prevalence of DLD and 50% DLD in prisoners*|
The so-called base rate fallacy is a common error in logical reasoning. It seems natural to conclude that if A is associated with B, then B must be associated with A. Statistically, that is true, but if A is extremely rare, then the likelihood of B given A can be considerably less than the likelihood of A given B.
So I don't think therefore that we need to seek explanations for the apparent inconsistency that's being flagged up on Twitter between rates of incarceration in studies of those with DLD, vs rates of DLD in those who are incarcerated. It could just be the consequence of the low base rate of incarceration.
Anderson et al (2016) Language impairments among youth offenders: A systematic review. Children and Youth Services Review, 65, 195-203.
Norbury, C. F., et al. (2016). The impact of nonverbal ability on prevalence and clinical presentation of language disorder: evidence from a population study. Journal of Child Psychology and Psychiatry, 57, 1247-1257.
*An R script for generating this figure can be found here.
Postscript - 4th November 2017
The Twitter discussion has continued and drawn attention to further sources of information on rates of language and related problems in prison populations. Happy to add these here if people can send sources:
Talbot, J. (2008). No One Knows: Report and Final Recommendations. Report by Prison Reform Trust.
House of Commons Justice Committee (2016) The Treatment of Young Adults in the Criminal Justice System. Report HC 169.