There are various factors which have been studied that indicate that fathers are not good as primary attachment. Fathers do not produce oxytocin which mothers do, which triggers nurturing behaviour that makes women biologically predisposed to being the primary attachment figure. Fathers are less likely to be good at the nurturing role as mothers are and therefore cannot form as intense attachments. Fathers also tend to be less-responsive to cues and distress signals, as found by Hrdy (distress levels) and Heerman (cues)