Most creatures on this earth are polygamous, why then are modern humans for the most part monogamous? Is monogamy useful in society? Does monogamy or polygamy have a basis in biology? Is there some normative fact about monogamy/polygamy? How come so many relationships are monogamous in name only? Ken introduces Helen Fisher, Research Professor and member of the Center for Human Evolutionary Studies at Rutgers University.

John mentions that many people want committed monogamous relationships while still having adventurous sexual lives, is there a biological basis for this tension? Fisher identifies three different brain systems that humans have evolved for mating purposes: the sex drive, romantic love, and the sense of attachment. Fisher goes on to claim that these distinct systems can interact or act separately from one another.

John asks about the seemingly vague notion of "romantic love" and Helen Fisher responds with a categorization of romantic love as a universal phenomenon across cultures that begins with the loved-one taking on special meaning and focus; an intense increase in energy; mood swings and dependence on the loved one; and more than any other characteristic--obsession. From this universal categorization Helen Fisher concludes that romantic love originates in specific brain circuitry.

Ken wonders why we would ever evolve to be so conflicted, Fisher responds that we didn't evolve to be happy, we evolved to reproduce! Ken and Helen discuss whether men and women are wired differently in this regard. John pushes for a definition of monogamy and Helen Fisher discusses monogamy around the world. Ken wonders why our culture and our biology seem so conflicted, Helen uses our anthropological history to explain this dichotomy. John and Ken discuss monogamy in pop and high cultures, and Helen Fisher discusses society's reasons for imposing rules of sexuality. Fisher explains the societal and philosophical justifications for our disapproval of adultery. Helen Fisher's primary argument for the evolution of monogamy is that when species have young which are susceptible to harm they need a pair of parents to protect them through infancy: humans, wolves, most birds. Ken and John seem to agree that the three-part definition of relationships is a very powerful intellectual tool for thinking about marriage and other concepts.

• Roving Philosophical Report (Seek to 04:24): Polly Stryker interviews swingers in "lifestyle clubs" about marriage, the bounds of monogamy, love, and sex.

• Conundrum: Alzheimer Case (Seek to 46:19): John and Ken discuss giving prescription memory drugs to Alzheimer's cases whose cognitive functioning is severely impaired, but who are still able to realize the burden they place on others and the disappointment of their old goals in life.