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Hospitalization rates for mental health conditions among children have been increasing locally and nationally in recent decades. This increase cannot be explained in terms of individual-level risk factors alone, suggesting short and long-term impacts of contextual factors.  Previous research has shown children who experienced major natural disasters, war and terrorism in the United States and around the world have higher risk of adverse mental health outcomes among children (Giacaman, Shannon, Arya, Saab, & Boyce, 2007; Hoven, Duarte, Lucas, & et al., 2005; Hoven, Duarte, & Mandell, 2003; Somasundaram, 2003).  These mass trauma occurrences are also known as collective trauma events, “a traumatic event that affects an entire society” (Hirschberger, 2018), resulting in similar psychological responses of those exposed both indirectly and directly.

Children in New York State have witnessed several collective traumatic events during these last two decades including the September 11 attacks, the Great Recession, and Hurricane Sandy and its aftermath. The September 11 attacks, a series of four coordinated terrorist attacks including the destruction of the World Trade Center towers in New York City, occurred in 2001.  The attacks killed 2,996 people that day (Arias, Anderson, Kung, Murphy, & Kochanek, 2003), and left an indelible impact on the American psyche.  The Great Recession from 2008-2010 was the deepest and longest economic decline in the postwar era (Elsby, Hobijn, & Şahin, 2010). Besides these man-created disasters, children in the New York area also experienced Hurricane Sandy in October 2012.  Hurricane Sandy caused massive disruptions in power and medical care (Ben-Ezra et al., 2014) and devastated the neighborhoods and lives of many New Yorkers (Powell, Hanfling, & Gostin, 2012).

Our study examined the trend in children’s mental health hospitalization rates from 1999-2013 using an age, period, and cohort (APC) analysis.  APC analysis detects variations in time trends associated with specific ages, variations associated with specific years, and variations associated with year of birth.  APC analysis distinguish the influence of collective traumatic events experienced by children during a sensitive developmental period (cohort effect) from the influence of these events on all children independent of birth cohort (period effect) and changes in mental health risk associated with age (age effect).  In our recent article in the Journal of Traumatic Stress (Liu & Lim, 2019), we were specifically interested in identifying possible cohort effects due to collective traumatic events.  Cohort effects may occur if a collective trauma event differentially affects mental health hospitalization depending on age of exposure.   

We found mental health hospitalization rates increased with age from five through 15 years of age.  We also found period effects in 2004 and 2013, which had higher mental health hospitalization rates than the expected for the specific years corresponding to approximately three years after the September 11 attacks and the Great Recession. In addition, our results indicate a birth cohort effect for children born in 1992-1995.  The birth cohort effects that we found suggest children who experienced the September 11 attacks during middle childhood and the Great Recession during puberty are at increased risk of mental health hospitalizations compared with other birth cohorts.

Our results suggest that these large-scale collective traumatic events in the past two decades might have contributed to children’s mental health hospitalization time trends in New York State. This study should be replicated using other geographically-specific collective traumatic events. It will be important to understand the role of collective traumatic events in driving the increase in local and national mental health hospitalization rates among children. In so doing, researchers and practitioners may consider strategies and interventions to address short and long-term mental health problems among high-risk subpopulations of birth cohorts following collective traumatic events. 


Arias, E., Anderson, R. N., Kung, H.-C., Murphy, S. L., & Kochanek, K. D. (2003). Deaths: final data for 2001. National vital statistics reports, 52(3), 1-116.

Ben-Ezra, M., Goodwin, R., Palgi, Y., Kaniasty, K., Crawford, M. Z., Weinberger, A., & Hamama-Raz, Y. (2014). Concomitants of perceived trust in hospital and medical services following Hurricane Sandy. Psychiatry Research, 220(3), 1160-1162. doi:https://doi.org/10.1016/j.psychres.2014.08.012

Elsby, M. W. L., Hobijn, B., & Şahin, A. (2010). The Labor Market in the Great Recession.

Giacaman, R., Shannon, H. S., Arya, N., Saab, H., & Boyce, W. (2007). Individual and collective exposure to political violence: Palestinian adolescents coping with conflict. European Journal of Public Health, 17(4), 361-368. doi:10.1093/eurpub/ckl260

Hirschberger, G. (2018). Collective Trauma and the Social Construction of Meaning. Frontiers in Psychology, 9, 1441-1441. doi:10.3389/fpsyg.2018.01441

Hoven, C. W., Duarte, C. S., Lucas, C. P., & et al. (2005). Psychopathology among new york city public school children 6 months after september 11. Archives of General Psychiatry, 62(5), 545-551. doi:10.1001/archpsyc.62.5.545

Hoven, C. W., Duarte, C. S., & Mandell, D. J. (2003). Children's mental health after disasters: the impact of the World Trade Center attack. Curr Psychiatry Rep, 5(2), 101-107.

Liu, SY. Lim, S. (2019). Age, period and cohort effects in children’s mental health hospitalization rates in New York State from 1999-2013. Journal of Traumatic Stress. Advance online publication.

Powell, T., Hanfling, D., & Gostin, L. O. (2012). Emergency Preparedness and Public Health: The Lessons of Hurricane SandyEmergency Preparedness and Public Health. JAMA, 308(24), 2569-2570. doi:10.1001/jama.2012.108940

Somasundaram, D. (2003). Collective Trauma in Sri Lanka. Intervention: International Journal of Mental Health, Psychosocial Work and Counselling in Areas of Armed Conflict, 1(1), 1-11.

Reference Article:

Liu, S.Y. and Lim, S. (2020), Collective Trauma and Mental Health Hospitalization Rates Among Children in New York State, 1999–2013: Age, Period, and Cohort Effects. Journal of Traumatic Stress. doi:10.1002/jts.22449

Discussion Questions:

  1. Are there other political or cultural events in the last two decades that you would consider to be a collective traumatic event with possible age-period-cohort effects on mental health hospitalization?
  2. What are plausible public health interventions following collective traumatic events to help mitigate period and cohort effects on mental health hospitalizations?

About the Authors:

Sze Yan Liu was a senior epidemiologist at the New York City Department of Health and Mental Hygiene at the time of this study. She is currently an assistant professor at Montclair State University.

Sungwoo Lim is the Director of the Research and Evaluation at the Bureau of Epidemiological Services at the New York City Department of Health and Mental Hygiene.