The findings in this study mostly support our hypotheses. The prevalence rates of ID and ADHD were both higher than those reported in the general population from normative (general population) data. With regard to ADHD, rates were marginally higher for childhood symptoms during a period of civil unrest, which suggests that rates of ADHD in custody may be influenced by local and situational factors for young detainees. As expected, there were high rates of co-morbidity, especially between ADHD and CD, which are known to occur together at a higher rate than chance . We found the shared variance of CD with childhood and current ADHD symptoms to be 59% and 44%, respectively, and CD was associated with ID because of its comorbidity with ADHD.
Our results indicate that ADHD and CD symptoms predicted consumption of staff time, with ADHD symptoms driving requests of staff time after controlling for CD and duration of time spent in custody. One possible explanation for this is that those who are symptomatic for ADHD have increased behavioral disinhibition (for example, ) where impatience and restlessness may be expressed as requests being made of staff. It is not possible to comment on the legitimacy of the requests made or the time taken to deal with each request. This would be important to determine in future research as the apparent increased demand of people with ADHD in restricted custodial settings has staffing and resource implications. ID was not significantly correlated with requests for staff time, which may have been due to the fact that very few of the detainees performed poorly on the LDSQ. This supports the findings of Gudjonsson and colleagues from the Royal Commission Study [2, 3] and the view of Murphy and Mason  that very few people with severe ID are likely to have contact with the CJS. An alternative explanation for the non-significant relationship is that individuals with intellectual disability may be particularly passive (for example, ) and not make requests of staff.
The exploratory analysis found that the implementation of improved assessment procedures had not increased the rate of the use of Appropriate Adults beyond that reported 20 years previously. Indeed, only two of the detainees screening positive for ID and one for ADHD had been provided with an Appropriate Adult. This represents a serious flaw in the current risk assessment process, especially given the recent initiative of HCPs being present at police custody suites.
The indicated rate of ID in the current study (6.7%) is consistent with previous findings  and suggests an increase from that found in the general population (for example, ). The rate is substantially lower than that reported among offenders in the Søndenaa et al. study , but it is consistent with that reported in the Bradley Report . Research has shown that many people who have had their conviction overturned on appeal are of low intelligence [7, 13, 56–58]. Therefore, early identification of their vulnerabilities may prevent wrongful convictions . The same holds true for people with ADHD. They are even less readily diagnosed than those with ID, in spite of the condition being more common, and the vulnerabilities associated with their condition are not so well established within the CJS . In the current study, the detainees with ADHD showed almost a 10-fold increase from the adult general population when screens were used and about a 7-fold increase when a diagnostic interview was used. We found that the rate of ADHD was three to four times higher than that for ID, which suggests that there are going to be many more detainees at police stations with ADHD than ID. Therefore, appropriate screening for people with ADHD should in future be incorporated into the routine screening and HCP assessment and the findings appropriately used to inform a decision on the need for an Appropriate Adult.
Although every individual brought into custody completed a risk-assessment (completed by themselves and by a Custody Officer), and almost half (44.7%) of this sample were interviewed by a HCP, very few were identified as having difficulties that merited the need for an Appropriate Adult. This under-identification has been recognized as one of the main barriers to providing adequate support (for example, ). Nevertheless, it is positive that items of the current risk assessment tool show promise for the identification of self-reported mental health problems and officer-reported self-harming behaviors in people with ADHD. This supports the work of Clare and Gudjonsson , highlighting the importance of self-reported vulnerabilities by persons detained at police stations as a potential ‘red flag’ for an Appropriate Adult.
Disappointingly, of those detainees interviewed by police only 4.2% were provided with an Appropriate Adult and this figure is almost identical to the 4.3% reported over 20 years previously in the Royal Commission study . This shows that the current risk assessment practices are failing and/or are not influencing the behavior of the police in terms of required provision of Appropriate Adults. Some of the risk assessment indicators, such as serious reading and writing problems, should have alerted the police and the HCP staff to the need for an Appropriate Adult. This suggests that the HCP staff employed at the police station do not focus sufficiently on psychological or mental health symptoms, including those associated with ADHD and ID. These individuals require an Appropriate Adult during police interviews and other formal procedures (for example, reading and signing documents), which they do not receive. This practice needs to change.
The findings should be considered in light of some particular strengths and limitations of the study. Although only representing one large Metropolitan Police station, the findings are likely to generalize to other police stations , but some regional variations may exist for the custodial and interview process . The study also has merit in the large sample of detainees who participated, and our power calculations indicated that the study was well-powered to screen for ID but marginally underpowered to screen for ADHD. Nevertheless, this is the first study of ADHD prevalence and associated behavior in police custody that has included a full clinical diagnostic methodology to estimate ADHD prevalence. The researchers were not qualified healthcare practitioners in diagnosing ADHD. However, they were trained to a level of good reliability by qualified clinical practitioners with expertise in the assessment of ADHD in adulthood and using the DIVA semi-structured interview.
Other than the prevalence data for ADHD using a clinical diagnostic methodology, all other analyses were conducted based on a sample obtained from screening rates of ADHD, CD and ID. Hence, diagnostic rates cannot be estimated for CD and ID as the number of false positive and false negative identifications are unknown. In clinical practice, for example, a diagnosis of ID would require assessment of both cognitive and adaptive functioning, and an assessment of childhood onset. Secondly, the screening and diagnostic data were based on self-reported information. This was a necessary methodological limitation as the high turnover of detainees in the custody suite and the short periods of time they generally spend in custody limited the opportunity for obtaining supplementary informant information. Nevertheless, adults with ADHD have been found to be reliable in reporting attention problems . There was also no detailed clinical interview of current symptoms relevant to mental illness, such as depression. Gudjonsson et al.  found that agitation and depressive symptoms were commonly found in a police station sample. Furthermore, anxiety and depression among suspects detained for interviews have been associated with the reporting of false confessions .
Finally, the focus of the current study has been on identifying vulnerabilities considered relevant to police custody procedures in relation to safeguards and reliability of police interviews. However, a further benefit of thorough screening procedures would be to signpost individuals who may require more detailed assessments for support within designated external services.