Transgenerational trauma, or intergenerational trauma, is the psychological and physiological effects that the trauma experienced by people has on subsequent generations in that group. The primary mode of transmission is the shared family environment of the infant causing psychological, behavioral and social changes in the individual.

Collective trauma is when psychological trauma experienced by communities and identity groups is carried on as part of the group's collective memory and shared sense of identity. For example, collective trauma was experienced by Jewish Holocaust survivors and other members of the Jewish community at the time, by the Indigenous Peoples of Canada during the Canadian Indian residential school system and by African Americans who were enslaved. When this collective trauma affects subsequent generations, it is called transgenerational trauma. For example, if Jewish people experience extreme stress or practice survivalism out of fear of another Holocaust, despite being born after the Holocaust, then they are experiencing transgenerational trauma.

Transgenerational trauma can be a collective experience that affects groups of people who share a cultural identity (e.g., ethnicity, nationality, or religious identity). It can also be applied to single families or individual parent–child dyads. For example, survivors of individual child abuse and both direct survivors of the collective trauma and members of subsequent generations individually may develop complex post-traumatic stress disorder.^{[citation needed]}

Examples of this include collective trauma experienced by descendants of the Atlantic slave trade; segregation and Jim Crow laws in the United States; apartheid in South Africa; the Scramble for Africa, Armenian genocide survivors, Jewish Holocaust survivors and other members of the Jewish community at the time; Bosnian war survivors; by the First Peoples of Canada during the Canadian Indian residential school system; by Native Americans when they were forcibly displaced and removed from their land; and, in Australia, the Stolen Generations and other hardships inflicted on Aboriginal and Torres Strait Islander peoples. Descendants of survivors may experience extreme stress, leading to a variety of other consequences.^{[citation needed]}

While transgenerational trauma gained attention in recent decades, the hypothesis of an epigenetic mechanism remains controversial due to a lack of rigorous experimental results on humans.

This field of research is relatively young, but has been expanded since the mid-2000s. Intergenerational trauma was first recognized in the children of Holocaust survivors. In 1966, psychologists began to observe large numbers of children of Holocaust survivors seeking mental help in clinics in Canada. The grandchildren of Holocaust survivors were overrepresented by 300% among the referrals to a psychiatry clinic in comparison with their representation in the general population. Since then, transgenerational trauma has been noted amongst descendants of African-Americans forced into slavery, Native American genocide survivors, war survivors, refugees, survivors of domestic violence, and many other groups that have experienced collective distress.

Research on possible biological mechanisms for inheritance of trauma began in the late 1990s. It has been suggested that traumatic stress can be passed down to future generations via epigenetics. However, the effect is difficult to separate from environmental and cultural transmission and conclusive evidence that it occurs in humans has yet to be found.

Although methylation of stress-related genes in humans may affect development, there is no evidence that these changes in humans are passed on to subsequent generations. Methylation is normally erased when an egg cell is fertilized.