	A 2014 analysis from the journal "Neuroscience & Biobehavioral Reviews" also found that there are sex differences in empathy from birth, growing larger with age and which remains consistent and stable across lifespan. Females, on average, were found to have higher empathy than males at all ages, and children with higher empathy regardless of gender continue to possess high empathy throughout development in life. Further analysis of brain tools such as event related potentials found that females who viewed human suffering had higher ERP waveforms than males, an indication of greater empathetic response. Another investigation with similar brain tools such as N400 amplitudes found higher N400 in females in response to social situations which then positively correlated with self-reported empathy. Structural fMRI studies have also found females to have larger grey matter volumes in posterior inferior frontal and anterior inferior parietal cortex areas which have been correlated with mirror neurons indicated by the fMRI literature. Mirror neurons are crucial for many if not most aspects of empathy. Females were also found to have a stronger link between emotional and cognitive empathy. The researchers use "The Primary Caretaker Hypothesis" to explain the stability of these sex differences in development. According to the hypothesis, prehistoric males did not have the same selective pressure as women and this led to sex differences in emotion recognition and empathy.