Developmentally Immature Children and Young People: Understandings, Neuroscience & Criminal Responsibility

  • Hannah Wishart

Student thesis: Phd


In England and Wales, developmental immaturity is not recognised as a factor deserving of exculpation from criminal responsibility for those who suffer from it and have a complete lack of capacity or a substantially impaired capacity. The explored cognitive scientific research reveals that developmental delays resulting in cognitive impairment can profoundly alter the brain’s structure and function that support a child/young person’s mental competence and behaviour. Developmental immaturity has been found to negatively affect their capacities to resist peer influences, foresee risks, think about the long-term consequences of their actions, control their emotions and regulate their impulsive actions. Yet, developmental immaturity is not uniformly represented in English law; weight is attached to its impact on the child/young person’s culpability in mitigation for sentencing. However, developmental immaturity does not form the basis of excuse from criminal responsibility in England and Wales. The thesis will show that the defence of diminished responsibility supported developmental immaturity as the basis of a partial excuse for individuals who suffered from it and killed under Section 2 of the Homicide Act (1957). However, under Section 52 of the Coroners and Justice Act (2009), those with developmental immaturity are excluded from successfully pleading the defence because it falls outside the scope of a “recognised medical condition.” The empirical case study and examination of the “expert” problem in the thesis will reveal that the definitional requirements of Section 2 as amended detrimentally impact developmentally immature children/young people who kill (aged 10 to 17 years of age). They are convicted of murder and sentenced to a starting point of 12 years in detention because Section 2 as amended is unavailable to them as a defence at trial. In the long term, those who suffer developmental immaturity and kill without “a recognised medical condition” will be convicted of murder unless specific developmental immaturity defences are introduced into English law, offering individuals with developmental delays and cognitive impairment an excuse from criminal responsibility. The thesis concludes by setting out two developmental immaturity defences that might help address the “expert” problem for developmentally immature persons who kill. It will also provide final remarks about which future diagnostic tools are promised to provide more accurate methods of diagnosis of medical conditions to plead the defence of diminished responsibility. The main contribution is the identification of a subgroup of children/young people who cannot successfully plead the defence of diminished responsibility. Neuroscientific evidence is vital in identifying these persons and supporting the argument that developmental immaturity should form the basis of specific defences for those who suffer from it and commit crimes.
Date of Award31 Dec 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPhilip Handler (Supervisor)


  • children
  • developmental immaturity
  • criminal responsibility
  • Neurolaw
  • murder

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