The effects of traumatic early life experiences on adult behaviour can be transmitted to the next generation through unknown mechanisms. The authors show that male mice exposed to early life stress exhibit altered behavioural responses, as do their offspring. The sperm of the traumatized mice exhibited changes in the levels of small non-coding RNAs. Injection of RNAs purified from this sperm into wild-type oocytes produced offspring with behavioural alterations similar to those observed in traumatized mice, indicating that sperm RNAs contribute to the transgenerational inheritance of the effects of early life trauma.