Relationship infidelity is widespread in this day and age, but it has also been present throughout pretty much all of recorded history. Research suggests that part of the reason infidelity has been so common and so consistent throughout the years is that the human brain structure makes infidelity not only possible, but probable. We tend to think of love as a single concept or experience, and this belief is part of why monogamy has become the accepted societal standard in nearly every modern and historical human society. However, there are actually three different brain systems that contribute to what we think of as love, according to biological anthropologist Helen Fisher. Romantic Love, Partner Attachment and Sexual Desire First, we have a brain system that experiences romantic love and the desire to focus courtship and mating energy on a single person. The fact that human beings experience romantic love partly explains why monogamous unions have become the standard practice in societies worldwide. The partner attachment and bonding that motivates couples to remain together for mutual comfort and for support raising a child is another brain system. Fisher suggests that while romantic love developed primarily as a way to focus mating energy, partner attachment developed as a way to ensure that pairs remain together through the process of raising a child at least through infancy. Finally, there is perhaps the most fundamental brain system of all\u2014and one that every species on earth that reproduces through mating shares. This of course is the sex drive, which encourages human beings and other species to mate with a range of partners in order to produce as many offspring as possible with as many strong genes as possible. The brain systems related to romantic love and partner attachment likely evolved as a way to keep the sex drive in check and to help more offspring to be reared successfully. Brain Can Feel Different Kinds of Love for Different People These different brain systems can work together and combine to form a strong attraction and attachment to one person. However, these systems can also work independently or in various combinations, and this may contribute to infidelity. People can easily be in love with one person while feeling sexually attracted to another person or have a generally strong sex drive that makes them desire other partners. Some long-term couples may have strong partner attachment but no longer feel romantic love for one another. Plenty of people feel strongly sexually attracted to certain people, and even fall in love, but never develop the strong partner attachment that sustains a relationship for the long term. It is also possible for someone to be strongly attached to his or her partner while in love with another person and also driven toward still more sexual partners. However, the culture of monogamy expects people to feel romantic love and strong attachment to one person and to be either sexually satisfied with that one person or able to easily control their sex drives thanks to the influence of love and attachment. This certainly happens for some people, but it just as certainly doesn\u2019t happen for others. When it doesn\u2019t happen, relationships may fall apart altogether or infidelity may occur. Of course, different brain systems are not the only factor in infidelity, and infidelity is certainly not a foregone conclusion when these brain systems no longer inspire feelings for just one person. But thinking of love as a single, all-inclusive experience may leave us unprepared and vulnerable to poor choices when these systems stop working in sync.