In this review we argue that relatively recent evolutionary adaptations that are relational or psychological in nature might refocus, dampen, or otherwise shape hormonal processes related to evolutionarily Bolder^behaviors. We focus on the steroid hormones testosterone and estradiol and discuss a) their associations with Bolder relational processes^such as mate competition, dominance and nurturance, and b) the ways in which Bnewer relational processes^such as commitment and attachment relate to these hormones in the context of intimate relationships. We propose that these new relational processes might influence hormones in a manner that enables short-term mating relationships to be transformed into long-term romantic pair-bonds that may be unique to humans.Keywords Mating . Pair-bonds . Testosterone . Estradiol . Evolution . Adaptations Most species, including humans, have evolved some repertoire of behaviors that facilitate short-term mating (i.e., uncommitted sexual relationships that last for a short period of time, Simpson and Gangestad 1991), such as attracting desirable mates, competing for the attention of those mates, and keeping others away from mates once they have been acquired (Gangestad and Simpson 2000). These evolutionarily Boldb ehaviors occur frequently, can be engaged with limited conscious effort (Miller 1998), and are important components of the relationship initiation process.Adaptive Human Behavior and Physiology (2015) 1:150-176 DOI 10.1007/s40750-015-0021-9 Britney M. Wardecker and Leigh K. Smith are co-first authors and contributed equally to this work. Only a few species, however, have evolved behaviors designed to deliberately maintain long-term, sexually exclusive relationships. Fewer than 5 % of mammalian species establish long-term, monogamous pair-bonds with adult mating partners (Kleiman 1977). A pairbond can be defined as an enduring and close relationship between two adult members of a species that generally involves sexual activity, intimacy, the desire to be close to one another, and deep emotional investment (Hawkes 2004; van Anders et al. 2011). Among humans, the formation of these types of pair bonds is a meaningful priority. For example, forming intimate connections is reported as one of the most important human motives, sometimes even more so than is meeting basic physiological needs such as securing food and water (Kenrick et al. 2010). Indeed, there may be several benefits to successfully establishing a long-term pair-bond. Pair-bonded partners benefit from increased paternal investment, which in turn promotes offspring health (Barash 1977;Fisher 1998;Kenrick and Trost 1997). The formation of long-term pair-bonds may also decrease the risk of contracting sexually transmitted diseases (Campbell and Loving, in press), and partners who are married or committed to long-term relationships tend to live longer than their unmarried and single counterparts (Drefahl 2012;Van Poppel and Joung 2001).Yet, transforming short-term mating relationships into long-term pair...