According to the 2015 "International Encyclopedia of the Social & Behavioral Sciences", sex differences in aggression is one of the most robust and oldest findings in psychology. Past meta-analyses in the encyclopedia found males regardless of age engaged in more physical and verbal aggression while small effect for females engaging in more indirect aggression such as rumor spreading or gossiping. It also found males tend to engage in more unprovoked aggression at higher frequency than females. This analysis also conforms with the "Oxford Handbook of Evolutionary Psychology" which reviewed past analysis which found men to use more verbal and physical aggression with the difference being greater in the physical type. There are more recent findings that show that differences in male and female aggression appear at about two years of age, though the differences in aggression are more consistent in middle-aged children and adolescence. Tremblay, Japel and Pérusse (1999) asserted that physically aggressive behaviors such as kicking, biting and hitting are age-typical expressions of innate and spontaneous reactions to biological drives such as anger, hunger, and affiliation. Girls' relational aggression, meaning non-physical or indirect, tends to increase after age two while physical aggression decreases. There was no significant difference in aggression between males and females before two years of age. A possible explanation for this could be that girls develop language skills more quickly than boys, and therefore have better ways of verbalizing their wants and needs. They are more likely to use communication when trying to retrieve a toy with the words "Ask nicely" or "Say please."