How can judges use social science evidence to inform sentencing? A dialogue between legal research and practice

This series is published in collaboration with the CORD Partnership.

Social science evidence and sentencing in Australia and New Zealand – Lynn Clarke-Hearty

At a recent presentation at Dublin City University, Prof. Kris Gledhill, a legal researcher from Auckland University of Technology, presented fascinating research on the use of social science evidence in Australasia in serious cases that considered aggravating and mitigating factors. He proposed that we can learn much from Australia and New Zealand in this regard, given our shared common law systems and postcolonial contexts.

Sentencing processes are becoming more structured in common law jurisdictions, including through legislation. For example New Zealand’s Sentencing Act 2002 defines the purposes for which a sentence may be imposed, without dictating the weight to be given each purpose. Examples include offender accountability, deterrence, victims’ interests, public protection, rehabilitation and reintegration. The legislation outlines a sentencing hierarchy and factors (such as the offender’s personal circumstances) and aggravating/mitigating circumstances for judges to consider. Judges can also facilitate dialogue between prosecution and defence counsel, promote proportionate and consistent sentences and use restorative justice. A presumption against imprisonment is provided generally ‘to the desirability of keeping offenders in the community as far as that is practicable and consonant with the safety of the community’, and specifically where sentencing prioritises restitution or rehabilitation.

How, within this already complex decision-making landscape, can evidence from social sciences inform sentencing decisions? Although Gledhill presented several examples, this piece focuses on the themes of youth and addiction and drug dealing.

Sentencing youth

Gledhill explored youth as a mitigating factor. In Churchward [2011] NZCA 531, which concerned an appeal against a life sentence and the minimum period of imprisonment (MPI) of a murder conviction, the Court reduced the MPI from 17 to 13 years, accepting that a 17-year MPI would be “manifestly unjust” for the appellants, aged 17 and 14. Acknowledging age-related neurological differences, psychosocial and emotional factors and mental health, the Court determined that “the process of ‘finding oneself’ tends to lend itself to experimentation which, for some adolescents in unstable environments, means engaging in risky activities”. Reaffirmed in Dickey v R [2023] NZCA 2, the Court of Appeal reduced the MPI for each appellant, showing a move beyond the simple statement that age is relevant, but explaining its relevance and applying such evidence with impact.

Unlike New Zealand, Ireland does not permit mandatory life imprisonment for minors convicted of murder. Additionally, the Criminal Justice Act 1990 s.2 was amended recently per Amah and Musueni v. Ireland Ors (AG and APP) [2024] IEHC 523, which disapplied the mandatory life sentence for murder for persons who ‘age out’ during the trial or sentencing process. Therefore, it is not controversial that youth is a mitigating factor. Applying social science evidence here contextualises offending, ensuring sentencing and decision-making are holistic and transparent.

Addiction, drug dealing and the limits to deterrence

In Zhang v R [2019] NZCA 507, concerning methamphetamine dealing, the Court of Appeal accepted that deterrence through sentencing lacked impact. It was noted that rational choice was “constrained by mental disorder, addiction, poverty, duress or other supervening vulnerability”, with evidence in this and similar cases to support such claims. Despite legislative constraints, the Court stated that it is ‘these vulnerabilities’, for offenders with a limited role in a distribution chain, on which “enlightened sentencing policy should focus”.

Updating Zhang, Berkland v R, Harding v R [2022] NZSC 143 discussed the issue of “causation” and the offender’s background. The Supreme Court determined that background factors providing a “causative contribution” and a “rational explanation” for offending can be taken into account. Citing social science research on mental illness, addiction and deprivation, the Court asked judges to “reflect on the power of background in the shaping of life opportunities and beware of imposing unrealistic expectations in hindsight”. According to the Court, background is not causative of offending, but correlation becomes a “powerful explanatory force” which cannot be ignored.

What can Ireland learn?

The purposes and principles of the 2002 Act in New Zealand are to be interpreted as ‘guidelines, not tramlines’, preserving judicial discretion and proportionality. This enables transparency and balance in sentencing procedures. This approach would be useful in Ireland, particularly in the District Court which experiences the highest volume of low-level offences, and which sees many individuals whose behaviour reflects the vulnerabilities identified in Australasian case law.

Australasian courts have allowed social science evidence to be admitted to explore reduced culpability arising from complex factors. In the context of the Irish prison population and minors in detention, this approach would be favourable given the rise in justice-involved individuals, first-time and repeat, with mental ill-health, addictions and complex trauma. Harm and offending should not be excused or downplayed, but contextual background is a “powerful explanatory force”. Social science challenges the validity of assumptions about human behaviour featuring in political and jurisprudential theory, “including utilitarian aims of securing desistance from crime and deontological aims of responding to breaches of obligations to society” as noted by Gledhill.

If similar approaches were to be introduced through legislative intervention or use of social science in evidence, a collaborative and focused dialogue between criminal justice stakeholders will be required, including education on social science research, admittance of and regard for such evidence in court, and application in context of sentence hierarchies, with regard for judicial discretion. Stakeholders at different points of the criminal justice system could use such evidence to provide holistic, effective responses to crime, deliver proportionate sentences, promote accountability, protect victim and social interests, and reduce recidivism. In its absence, Ireland’s justice system will deepen harm and trauma for both victims and perpetrators, with a collateral impact on society.

A practitioner’s perspective on social science evidence in sentencing – Gemma McLoughlin-Burke BL

The sentencing regime in Ireland requires that a sentence must fit both the offence and the offender. In DPP v. McCormack [2000] 4 I.R. 356, the Court outlined ‘best practice’ as (i) identifying the headline sentence having regard to the available range for the offence, the seriousness of the offence and aggravating factors, and then (ii) reducing the sentence to account for mitigation, including the offender’s personal circumstances.

In DPP v. Tiernan [1988] IR 250, the Supreme Court expressed serious reservations about appellate Courts providing ‘tariffs’ for sentencing, however, in 2014, the Court of Criminal Appeal delivered three judgments (DPP v. Ryan [2014] 2 ILRM 98, DPP v. Fitzgibbon [2014] 2 ILRM 116 and DPP v. Z [2014] 2 ILRM 132) setting out sentencing guidelines for firearms offences, assault causing harm and rape. This practice has continued for other offences. Realistically, sentencing guidelines now form part of Irish law.

Despite the requirement to sentence both the offence and offender, and to balance rehabilitation, deterrence and punishment, there is an increased focus on imprisonment. This is evidenced by:

• (i) The ever-growing number of persons in custody, which increased by nearly 55 per cent between 2004 and 2024;
• (ii) The increasing time a person on a life sentence spends in prison, averaging 18 years between 2005 and 2014, and 24 years in 2023;
• (iii) The introduction and increased use of mandatory minimum sentences, including for firearms and drugs offences.

It is also this practitioner’s view from extensive review of decisions of the Criminal Court of Appeal that sentencing appeals taken by convicted persons are frequently unsuccessful, whereas leniency sentence appeals taken by the DPP are often successful.

The general demographic coming before the criminal courts are persons from socio-economically deprived areas who have disproportionately experienced trauma, physical and sexual violence, bereavement and addiction. This is reflected in statistics of those currently serving sentences in Irish prisons:

(i) The majority have never sat a State exam, and over half left school before the age of 15;
(ii) They are 23 times more likely to come from a seriously deprived area;
(iii) 70 per cent were unemployed at the time of imprisonment.

All of this evidences something that practitioners experience daily: too many criminal justice decision-makers in the criminal justice system prioritise punishment and, consequently, imprisonment, which disproportionately targets those from poor, disadvantaged areas.

Sentencing youth and drug addiction in Ireland

The recent decisions of the Supreme Court in DPP v. PB [2025] IESC 12 and DPP v. CC [2025] IESC 11 emphasised children’s lack of cognitive development, appreciation for the consequences of their actions and immaturity. Children are sentenced differently since the introduction of the Children Act 2001, but these judgments lay a new evidence-based emphasis on the need to prioritise rehabilitation for children. The impact of these decisions on the sentencing process is, as yet, unknown.

In DPP v. Fitzgibbon [2008] IESC 15, the Court of Criminal Appeal held that Courts must take account of serious drug addiction in sentencing. However, in my view, insufficient consideration is given to drug addiction. Often, directions are given that an individual engage with addiction services and, when they are unsuccessful or do not engage in a manner satisfactory to the Court, they are punished with custodial sentences. The entirely unrealistic expectation that a person with a life-long addiction will suddenly recover by threat of imprisonment sets people up to fail. Imprisonment is also not a solution, with rampant drug use in prisons and long waiting lists for drug addiction services.

This obsessively punitive approach to drugs is not only ineffective and disproportionate, it results in serious injustices. A truly disturbing example is DPP v. Nguyen [2014] IECA 55 where the DPP sought an increased sentence for two individuals who they agreed had been trafficked into this jurisdiction to work in a growhouse. The Court of Criminal Appeal increased their sentences to six years, with the final two years suspended. They reasoned that the Court must deter organisations from using trafficked persons in the drugs trade. These were victims of trafficking. This is not justice.

Suggested amendments

I am in favour of the codification of sentencing guidelines which include social science and evidence-based considerations for a Court. Such guidelines could rebalance the system towards rehabilitation, indisputably the most effective way to tackle recidivism and keep society safe. In my view, the current sentencing process does not meet the Constitutional requirement to sentence both offence and offender. Codifying the mandatory considerations by a Court would provide clarity and accountability in the sentencing process.

Lynn Clarke-Hearty is a PhD Candidate at University College Dublin School of Law. Gemma McLoughlin-Burke BL is a Barrister.