Neurodiversity, Expert Evidence and “More Onerous Custody”: Limits on Culpability Reduction in Unlawful Act Manslaughter Sentencing

1. Introduction

R v IGG ([2026] EWCA Crim 419, CA (Crim Div), 12 March 2026) concerns an appeal against sentence following the appellant’s conviction for manslaughter after she violently shook her 4½ month-old daughter, L, during the first Covid lockdown; L collapsed and died six days later.

The appellant (aged 23 at the time) lived with her partner (later co-defendant) and two children. She was tried with the co-defendant on manslaughter and, in the alternative, causing or allowing the death of a child contrary to s.5 of the Domestic Violence, Crime and Victims Act 2004. The appellant was convicted of manslaughter. The co-defendant was acquitted (the prosecution did not pursue manslaughter; and the jury acquitted him of the s.5 offence).

The Crown Court imposed 15 years’ imprisonment. The appeal focused not on categorisation under the manslaughter guideline, but on whether the sentencing judge gave adequate weight to mitigation—particularly a psychological report diagnosing autism and ADHD (AuDHD), youth/immaturity, Covid-related stress, delay, and the asserted increased harshness of imprisonment.

The Court also confirmed the continuing effect of an anonymity order made under section 45 of the Youth Justice and Criminal Evidence Act 1999, requiring anonymisation of the appellant to protect the identities of her other children.

2. Summary of the Judgment

The Court of Appeal dismissed the sentence appeal. It held that:

  • The sentencing judge was entitled to conclude that the appellant’s age, immaturity, and neurodiversity did not reduce culpability for violently shaking a baby, notwithstanding the psychologist’s assertions.
  • The judge was entitled to treat the expert report with caution where it was based in part on an account of events rejected by the jury and where the appellant maintained denial.
  • Any mitigation arising from professionals’ prior observations of her parenting was undermined by the finding of an earlier shaking episode.
  • The sentence of 15 years was within the appropriate range and not manifestly excessive.

Importantly, the Court accepted one general proposition advanced by neither side in absolute terms: it is legitimate to reduce a custodial sentence where imprisonment will be more onerous for this offender than for the general prison population, and this principle applies to long sentences as well as short ones. However, the Court emphasised that such mitigation may be of limited significance and noted that neurodiversity is unfortunately prevalent within the prison population.

3. Analysis

3.1 Precedents Cited

The judgment does not cite prior appellate authorities by case name. Instead, it is structured around the application of statutory provisions and Sentencing Council guidance. The key sources were:

  • The Sentencing Council guideline on Unlawful Act Manslaughter (used to determine category, starting point, and range).
  • The guideline on sentencing offenders with mental disorders, development disorders or neurological impairments (used to assess whether neurodiversity/mental conditions reduced culpability and/or affected mitigation).
  • The guideline on sentencing children and young adults (used to consider maturity and neurological development in 18–25 year olds).
  • section 45 of the Youth Justice and Criminal Evidence Act 1999 (anonymity).
  • s.5 of the Domestic Violence, Crime and Victims Act 2004 (alternative offence charged at trial; relevant context but not determinative on sentence for manslaughter).

These sources influenced the outcome by framing (i) the culpability assessment (awareness of risk), (ii) the evaluative question whether impairment materially connected to offending, and (iii) the proper approach to expert opinion in sentencing.

3.2 Legal Reasoning

(a) Guideline categorisation and range discipline

The appellant did not challenge the categorisation. The judge placed the offence in Category B under the Unlawful Act Manslaughter guideline (starting point 12 years, range 8–16 years) on the basis that she was or ought to have been aware of an obviously high risk of death or really serious harm.

(b) Aggravation driving the sentence towards the top of the range

The judge identified aggravating features that justified moving to the upper end, including:

  • History of violence: pathological evidence of prior shaking injury; the judge found the appellant responsible.
  • Physical suffering of the infant, both immediately and over the six days before death.
  • Particular vulnerability of the victim as a baby.
  • Presence of the toddler sibling (given limited weight).
  • Blame-shifting towards the co-defendant.

The Court of Appeal accepted that this aggravation legitimately took the case to the top of the guideline range.

(c) Neurodiversity and expert opinion: connection, not label

A central issue was whether AuDHD and associated vulnerabilities diminished culpability. The Court’s approach closely tracks the structure of the guideline on sentencing offenders with mental disorders, development disorders or neurological impairments:

  • Culpability reduction depends on a sufficient connection between impairment/disorder and the offending behaviour.
  • The sentencer must make an evaluative judgment on the evidence as a whole.
  • The court is not bound by expert opinion on culpability.

The Court endorsed the sentencing judge’s caution for two linked reasons:

  • Denial and disputed narrative: the appellant continued to deny causing the injuries; the report was based partly on an account rejected by the jury, limiting its usefulness in explaining the offending mechanism.
  • Inability to explain the violence: the judge concluded nothing in the report explained why the appellant shook the baby; the Court accepted he was entitled to reach that conclusion.

(d) Young adulthood and maturity: relevance is fact-sensitive

The appellant relied on the young adult guideline passage that 18–25 year olds may be less able to evaluate consequences, limit impulsivity, and limit risk-taking. The judge expressly considered youth and immaturity but held they did not reduce culpability here. The Court upheld that assessment, emphasising that:

  • The trial judge had observed the appellant’s evidence and was well placed to evaluate maturity and responsibility.
  • There was no basis to say she could not appreciate the danger of shaking a baby (indeed she accepted the contrary in evidence).

(e) Delay and “more onerous custody”

The Court largely agreed with the Crown that delay was substantially explicable (family proceedings; specialist evidence; contested case driven by denial). However, the Court made a notable clarification on mitigation:

It is appropriate to reduce a sentence of imprisonment if the offender’s personal circumstances would render custody more onerous than for the prison population generally; and that applies as much to a longer sentence as to a shorter one.

Even so, the Court regarded this factor as only of “some but limited significance” on the facts, including because neurodiversity is common in custody and because the case’s gravity and aggravation remained dominant.

3.3 Impact

The decision’s practical significance lies in three areas:

  • Expert reports in sentencing: Where an offender maintains denial and the report relies on a narrative inconsistent with the verdict, the sentencer may legitimately treat the report’s culpability opinions with caution. Diagnosis alone will not secure a culpability discount without a clear, evidenced connection to the offending act.
  • Neurodiversity and culpability: The case underscores that neurodevelopmental conditions may inform mitigation (including prison impact) but will not automatically reduce culpability, particularly in offences involving obvious risks (such as infant shaking).
  • “More onerous custody” applies to long sentences too: The Court expressly confirmed that differential custody impact can justify reduction even where the sentence is lengthy—though it may be given limited weight depending on prevalence, management within prison, and the seriousness of the offence.

4. Complex Concepts Simplified

  • Unlawful act manslaughter: A form of manslaughter where death results from a dangerous unlawful act (here, violently shaking a baby). Sentencing focuses heavily on risk awareness and harm.
  • Culpability vs mitigation: “Culpability” concerns how blameworthy the offender was when committing the act (which affects the category/range). “Mitigation” concerns personal and contextual factors that may reduce the final sentence within the range.
  • Connection requirement (for mental disorder/neurodiversity): A diagnosis matters for sentencing only if it is shown to have affected decision-making, judgment, understanding of consequences, or ability to control impulses at the time—linked to the offending behaviour.
  • “More onerous custody”: A principle that imprisonment may be harsher for some offenders because of personal characteristics (for example, certain disabilities or vulnerabilities). If properly evidenced, that can reduce sentence length, even for long terms.
  • Manifestly excessive: The appellate test is not whether the Court would have sentenced differently, but whether the sentence falls outside the reasonable range of sentences open to the judge.

5. Conclusion

R v IGG [2026] EWCA Crim 419 affirms a disciplined guideline approach to unlawful act manslaughter sentencing in infant death cases, where aggravation (including prior similar harm and prolonged suffering) can properly justify a sentence near the top of the range.

It also clarifies that while neurodiversity and young adulthood must be considered, they do not necessarily reduce culpability—especially where the offending risk is obvious and the expert opinion cannot reliably explain the offence. Finally, the Court’s express recognition that “more onerous custody” can mitigate even a long sentence provides a useful, careful statement of principle, albeit one applied with restraint on the facts.