Effect of gut microbiota early in life on aggressive behavior in mice

•Germ-free mice show more aggressive behavior than ex-germ-free mice.•Germ-free mice inoculated with ex-germ-free mice feces reduced aggression.•Brain dopamine levels were higher in germ-free than those in ex-germ-free mice.•The gut microbiota early in life has an effect on aggressive behavioral traits.•The gut microbiota can affect brain development and behaviors of the host.