Thorhildur Halldorsdottir, PhD

Associate Professor / Principle Investigator and Co-Founding Chair of NeSTED

Reykjavík University | Iceland

About

My research sits at the intersection of clinical psychology, behavioural genetics, psychophysiology, epidemiology and public health — with a primary focus on adolescence. My work focuses on understanding why mental health problems are increasing in young people, who is most vulnerable, and what interventions can help. Working in Iceland’s unique national-registry environment and collaborating across disciplines and countries, I aim to develop targeted prevention and personalized intervention strategies that are accessible to youth worldwide.

Research Themes

• Adolescent Mental Health - Risk & Resilience – investigating trajectories of well-being and the mechanisms underlying who thrives and who develops problems

• Biological Embedding of Stress – how genetics, epigenetics, neuro-physiology, and environment interact during sensitive developmental windows

• Digital Environments & Youth – the role of social media, screen use and digital ecosystems in shaping youth mental health

• Population Science & Translational Impact – leveraging national registries, longitudinal cohorts and multi-level modelling to inform policy and prevention

• Science Communication & Public Engagement – translating science into practice, policy, and public understanding

Selected Publications

Sigurdardottir, K. R., Hannesdottir, D. K., Hauksdottir, B., Ollendick, T. H., Davidsdottir, K., & Halldorsdottir, T. (2025). Youth without intellectual impairment who are autistic, ADHD or autistic-ADHD: Incidence, co-occurring psychiatric conditions and sex differences in Iceland. Lancet Child & Adolescent Health.

Halldorsdottir, T. (2025). Youth mental health crisis: A multidisciplinary view of the COVID-19 pandemic. Invited contribution to the “Future Leaders in Stress” special issue in Neurobiology of Stress.

Haskell, E., Sigmarsdottir, B., Thorisdottir, I. E., Valborgarson, A., Bonilla Aparicio, E., Kiviruusu, O., Suvisaari, J., Chang, Z., Ystrom, E., Butwicka, A., Asgeirsdottir, B. B., Valdimarsdottir, H. B., Sigfusdottir, I. D., Allegrante, J. P., & Halldorsdottir, T. (2025). Adolescent mental health before, during and after the COVID-19 pandemic in Iceland: A repeated, cross-sectional, population-based study. Lancet Regional Health – Europe.

Thorisdottir, I. E., Agustsson, G., Oskarsdottir, S. Y., Kristjansson, A. L., Asgeirsdottir, B. B., Sigfusdottir, I. D., Valdimarsdottir, H. B., Allegrante, J. P., & Halldorsdottir, T. (2023). Effect of the COVID-19 pandemic on adolescent mental health and substance use up to March 2022 in Iceland: A repeated, cross-sectional, population-based study. Lancet Child & Adolescent Health, 7(5), 347–357.

Jonsdottir, S. D., Hauksdottir, A., Aspelund, T., Jakobsdottir, J., Runarsdottir, H., Gudmundsdottir, B., ... Halldorsdottir, T., & Thordardottir, E. B. (2022). Risk factors for workplace sexual harassment and violence among a national cohort of women in Iceland: a cross-sectional study. Lancet Public Health, 7(9), e763–e774.

Thorisdottir, I. E., Asgeirsdottir, B. B., Kristjansson, A. L., Valdimarsdottir, H. B., Jonsdottir Tolgyes, E. M., Sigfusson, J., Allegrante, J. P., Sigfusdottir, I. D., & Halldorsdottir, T. (2021). Depressive symptoms, mental well-being and substance use among adolescents before and during the COVID-19 pandemic in Iceland: A longitudinal analysis. Lancet Psychiatry, 8(8), 663–672. doi: 10.1016/S2215-0366(21)00156-5

Halldorsdottir, T., Piechaczek, C., Matos, A. P., Czamara, D., Pehl, V., Wagenbüchler, P., Feldmann, L., Quickenstedt-Reinhart, P., Allgaier, A-K., Freisleder, F. J., Major Depressive Disorder Working Group of the Psychiatric Genomics Consortium, Greimel, E., Rex-Haffner, M., Arnarson, E. Ö., Craighead, W. E., Schulte-Körne, G., & Binder, E. B. (2019). Polygenic risk: Predicting depression outcomes in clinical and epidemiological cohorts of youth. American Journal of Psychiatry. doi: 10.1176/appi.ajp.2019.18091014

Halldorsdottir, T., Kurtoic, D., Mueller-Myhsok, B., Binder, E. B., & Clancy, B. (2019). Neurobiology of self-regulation: FKBP5 × early-life stress associated with emotional and cognitive development in childhood. American Journal of Psychiatry. doi: 10.1176/appi.ajp.2019.18091018

Halldorsdottir, T., & Binder, E. B. (2017). Gene–environment interactions: From molecular mechanisms to behavior. Annual Review of Psychology, 68, 215–241. doi: 10.1146/annurev-psych-010416-044053

Halldorsdottir, T., Ollendick, T. H., Ginsburg, G., Sherrill, J., Kendall, P. C., Walkup, J. T., Sakolsky, D. J., & Piacentini, J. (2015). Treatment outcomes in anxious youth with and without comorbid ADHD in the CAMS. Journal of Clinical Child and Adolescent Psychology, 44, 985–991. doi: 10.1080/15374416.2014.952008

Halldorsdottir, T., & Ollendick, T. H. (2014). Comorbid ADHD: Implications for the treatment of anxiety disorders in children and adolescents. Cognitive and Behavioral Practice, 21, 310–322. doi: 10.1016/j.cbpra.2013.08.003