In the opening paragraph of Esther Perel's book, Mating in Captivity, she writes:"The story of sex in committed modern couples often tells of a dwindling desire and includes a long list of sexual alibis, which claim to explain the inescapable death of eros."It is this idea, that sexual desire dwindles when in a committed relationship, that Perel successfully tackles in her book. Popular perception suggests that committed relationships mark the end of sex. Yet research shows that when asked, many people indicate sexual desire as a key feature of romantic love.The work of myself and others in the field suggests to me that sexual desire ebbs and flows throughout life and relationships.Research by Murray and Milhausen (2012) recently tackled the length of relationship and desire connection, and found that length of relationship (in couples who were together for an average of 2 years) impacted sexual desire for women, but not men.In research by Klusmann (2002), men’s sexual desire tended to remain high while women’s sexual desire is found to decrease as early as one year into the relationship.In research I've conducted, I found that length of relationship (in couples who were together for an average of 4 years) didn't impact sexual desire for women or men, and women and men were equally likely to be the member of the couple with lower sexual desire relative to their partner. And in interviews with women in a relationship for a minimum of 5 years, myself and colleagues have found that there are a number of factors that impact the ebb and flow of sexual desire.Perhaps another reason the idea exists around sexual desire diminishing with length of relationship is the strong sexual desire in passionate love that is replaced by increased intimacy in companionate love (said to occur around two and a half years).All of this also makes me wonder, is it the relationship length that is decreasing the desire? Or simply the other milestones (kids, moving in, career moves) that happen to correspond to relationship length? And how do we keep the desire in our relationships over the long haul?Bringing it back to Mating in Captivity, where open and loving relationships are accompanied with dull sex lives, when we love someone, we feel responsible and secure. Responsibility and security clash with desire. So as the length of our relationship increases, we become closer to the individual, we have a greater sense of security, and we lose that animalistic sense of "throw down" that was such a large part of early sexual scripts in the relationship. As Perel puts it, "fire needs air, and many couples don't leave enough air."Creating that space, or "air", is perhaps one of the things that can be done in relationships when the desire is at a low ebb. But also just realizing that the ebbs of desire will be accompanied by upward flows is one way to ensure expectations for sex don't get in the way of pleasure from sex, especially in the context of long-term relationships.