Sociality and kleptoparasitism occur commonly in theridiid spiders. In both behaviors a number of conspecifics occupy a single web; gregariousness entails tolerance. Sociality has evolved several times in theridiids, but kleptoparasitism seems to have arisen only once. All four or more instances of sociality in theridiids are concentrated within a clade of relatively distal theridiids. This distribution of sociality suggests common cause, i.e. the presence of some characteristics that may facilitate the evolution of social behavior. The monophyletic genus Argyrodes, many of which are kleptoparasitic, is sister to the clade containing all social theridiids. Sociality and kleptoparasitism may thus be phylogenetically related in theridiid spiders; behaviors that facilitated the evolution of sociality could also have facilitated kleptoparasitism. Both may have their roots in maternal care.