Phyllis is a brunette with long hair. according to research by mackinnon et al. (2011), when phyllis enters a room of strangers, she is most likely to sit closer to

People with similar physical characteristics--to seek comfort people will find others who look similar to them to create immediate common ground.

This tendency in humans explains why people tend to seek out homogeneous groupings. We seek what we know and therefore we are most comfortable. This behavior can be exploited to create the 'isms' of society like racism or sexism.