Stephen S. Leff, PhD

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Stephen S. Leff, PhD, is a psychologist in the Department of Child and Adolescent Psychiatry and Behavioral Sciences and Co-Director of the Violence Prevention Initiative at Children's Hospital of Philadelphia.

Areas of Expertise: Aggression and bullying prevention programming, ADHD and peer relations, Community-based participatory research models for intervention and measurement development, Gender and social cognitive differences in aggression
Appointments and Referrals: 1-800-TRY-CHOP (1-800-879-2467)


Dr. Leff is a Professor of Clinical Psychology in Pediatrics and Psychiatry at the University of Pennsylvania School of Medicine and a psychologist at Children’s Hospital of Philadelphia. He is the Co-Director of the Violence Prevention Initiative at CHOP, and is a nationally recognized leader in aggression and bullying prevention programming, relational aggression (use of rumors and social exclusion to harm others), and the community-based participatory research (CBPR) process (working with community stakeholders to integrate their perspectives into best practice programming). He has been awarded six NIH grants, a National Science Foundation grant, a Department of Education grant, and a number of local foundation awards to support his aggression and bullying prevention programs in the urban schools. He lectures frequently both regionally and nationally, is on the Editorial Advisory Board for several leading journals, serves on federal Review panels, and is a member of the Pennsylvania Bullying Prevention Partnership and the National Bullying Research Network. He also is an attending psychologist in the ADHD Program at CHOP.

Education and Training


Philadelphia Child Guidance Center, Philadelphia, PA


Children's Seashore House of The Children's Hospital of Philadelphia, Philadelphia, PA

Undergraduate Degree

BA - Duke University, Durham, NC

Graduate Degree

PhD - University of North Carolina, Chapel Hill, NC

Titles and Academic Titles


Co-Director of the Violence Prevention Initiative at Children's Hospital of Philadelphia

William H. Bennett Professor of Pediatrics, University of Pennsylvania Perelman School of Medicine

Professor of Clinical Psychology in Pediatrics and Psychiatry, Perelman School of Medicine at the University of Pennsylvania

Departments and Services

Research Interests

Aggression prevention programming
Gender and social cognitive differences in aggression
Social skills promotion




Mehari, K.R., Moore, W., Waasdorp, T.E., Varney, O., Berg, K., & Leff, S.S.: Cyberbullying Prevention: Insight and Recommendations from Youth, Parents, and Pediatricians. Child: Care, Health & Development. Page: 1-7, 2018 Notes: DOI:10.1111/cch.12569.


Leff, S.S. & Feudtner, C. : Tackling Bullying: Grounds for Encouragement and Sustained Focus. Pediatrics 139(6), June 2017 Notes: doi:


Leff, S.S., Waasdorp, T.E., & Paskewich, B.: The broader impact of Friend to Friend (F2F): Effects on teacher-student relationships, prosocial behaviors, and relationally and physically aggressive behaviors. Behavior Modification 40(4): 589-610, 2016.


Leff, S.S., Paskewich, B.S., Waasdorp, T.E., Waanders, C., Bevans, K.B., & Jawad, A.F.: Friend to Friend: A randomized trial for urban African American Relationally Aggressive Girls. Psychology of Violence 5(4): 433-443, 2015 Notes: Published online: doi:


Leff SS, Lefler EK, Khera GS, Paskewich B, Jawad AF: Preliminary examination of a cartoon-based hostile attribution bias measure for urban African American boys. American Journal of Community Psychology 49: 332-346, 2011.

Leff SS, Thomas DE, Shapiro ES, Wilson K, Paskewich B, Necowitz-Hoffman B, Jawad AF: Developing and validating a new classroom climate observation assessment tool. Journal of School Violence 10: 165-184, 2011. PMCID: PMC3106103.




Leff, S.S., Waasdorp, T.E, & Mehari, K. R.: An Updated Review of Existing Relational Aggression Programs. In S. M. Coyne, & J. M. Ostrov (Eds.), The development of relational aggression; (pp. 283-317, Chapter xiii, 348 Pages) Oxford University Press, New York, NY. 2018 Notes:

Leff, S.S., Streit, K.V., & Paskewich, B.S.: Supporting Urban Youth through Play: Facilitating Social and Emotional Skills. Promoting Prosocial Behaviors in Children through Games and Play: Making Social Emotional Learning Fun. Laura Nabors & Renee Hawkins (eds.). Nova Science, Page: 147-175, 2018.

Bradshaw, C. P., O'Brennan, L.M., Waasdorp, T.E., Pas, E., Blumenstyk, J., Bartolo, D., & Leff, S. S.: The New Frontier: Leveraging Innovative Technologies to Prevent Bullying. Cambridge Handbook of Violence Behavior and Aggression (2nd Ed.). A. Vazsonyi, D. Flannery, & M. Delisi (eds.). Page: 724-735, 2018 Notes: 2nd edition.


Waasdorp, T.E., Horowitz-Johnson, Z., & Leff, S.S.: Cyberbullying across the lifespan: Risk factors and consequences.  Handbook of Bullying Prevention: A Lifecourse Perspective.  C.P. Bradshaw (eds.). National Association of Social Workers (NASW) Press, Page: 69-80, 2017.


Leff SS, Waasdorp TE, Waanders C, Paskewich BS. Better understanding and intervening to prevent relational aggression. In: Weist MD, Lever NA, Bradshaw CP, Owens JS, editors. Handbook of School Mental Health: Research, Training, Practice, and Policy, 2nd ed. New York: Springer; 2014. p. 171-84.