Prisons, developmental language disorder, and base rates

There's been some interesting discussion on Twitter about the high rate of developmental language disorder (DLD) in the prison population. Some studies give an estimate as high as 50 percent (Anderson et al, 2016), and this has prompted calls for speech-language therapy services to be involved in the working with offenders. Work by Pam Snow and others has documented the difficulties of navigating the justice system if your understanding and ability to express yourself are limited.

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*

As you can see, according to this scenario, the probability of going to prison is much greater for those with DLD than for those without DLD (2.24% DLD vs 0.17% without DLD), but the absolute probability is still very low – 98% of those with DLD will not be incarcerated.

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.

References
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.

The allure of autism for researchers

Data on $K spend on neurodevelopmental disorder research by NIH: from Bishop, D. V. M. (2010). Which neurodevelopmental disorders get researched and why? PLOS One, 5(11), e15112. doi: 10.1371/journal.pone.0015112

Every year I hear from students interested in doing postgraduate study with me at Oxford. Most of them express a strong research interest in autism spectrum disorder (ASD). At one level, this is not surprising: if you want to work on autism and you look at the University website, you will find me as one of the people listed as affiliated with the Oxford Autism Research Centre. But if you look at my publication list, you find that autism research is a rather minor part of what I do: 13% of my papers have autism as a keyword, and only 6% have autism or ASD in the title. And where I have published on autism, it is usually in the context of comparing language in ASD with developmental language disorder (DLD, aka specific language impairment, SLI). And, indeed in the publication referenced in the graph above, I concluded that there was disproportionate amounts of research, and research funding, going to ASD relative to other neurodevelopmental disorders.

Now, I don’t want to knock autism research. ASD is an intriguing condition which can have major effects on the lives of affected individuals and their families. It was great to see the recent publication of a study by Jonathan Green and his colleagues showing that a parent-based treatment with autistic toddlers could produce long-lasting reduction in severity of symptoms. Conducting a rigorous study of this size is hugely difficult to do and only possible with substantial research funding.

But I do wonder why there is such a skew in interest towards autism, when many children have other developmental disorders that have long-term impacts. Where are all the enthusiastic young researchers who want to work on developmental language disorders? Why is it that children with general learning disabilities (intellectual retardation) are so often excluded from research, or relegated to be a control group against which ASD is assessed?

Together with colleagues Becky Clark, Gina Conti-Ramsden, Maggie Snowling, and Courtenay Norbury, I started the RALLI campaign in 2012 to raise awareness of children’s language impairments, mainly focused on a YouTube channel where we post videos providing brief summaries of key information, with links to more detailed evidence. This year we also completed a study that brought together a multidisciplinary, multinational panel of experts with the goal of producing consensus statements on criteria and terminology for children’s language disorders – leading to one published paper and another currently in preprint stage. We hope that increased consistency in how we define and refer to developmental language disorders will lead to improved recognition.

We still have a long way to go in raising awareness. I doubt we will ever achieve a level of interest to parallel that of autism. And I suspect this is because autism fascinates because it does not appear just to involve cognitive deficits, but rather a qualitatively different way of thinking and interacting with the world. But I would urge those considering pursuing research in this field to think more broadly and recognise that there are many fascinating conditions about which we still know very little. Finding ways to understand and eventually ameliorate language problems or learning disabilities could help a huge number of children and we need more of our brightest and best students to recognise this potential.