While it is becoming more common today for fathers to be a part of child birth and care, research estimates that only 5-10% of mammalian fathers are a part of routine care.  These sort of fathers, commonly referred to as biparental fathers, partake in typical parenting behaviors such as interacting with their offspring, transporting, defending, playing with, and warming their offspring more than half of the time.  What is  most interesting and apparent in biparental fathers is the very pronounced shift in the hormone levels that coinciding with parent behaviors.  It is from these studies it became apparent that fathers undergo significant hormonal changes throughout the pregnancy, birth, and postpartum periods as they experience fatherhood.  While it is not common knowledge, there are specific hormones associated with a father's behavior.  When looking at uninvolved fathers it is apparent the opposite may be true - that  paternal behaviors and hormonal changes are largely unchanged.  Father-infant interactions is key.  Testosterone, vasopressin, oxytocin, opioids, pheromones, and prolactin have been specifically noted to affect fathers.  

Testosterone

Physiologically, testosterone facilitates reproductive effort by promoting the generation of sperm and supporting the development of traits such as muscle mass and increased stature. In males, lower testosterone levels are associated with both mating effort and paternal investment (Gray & Campbell, 2009).   It is well-documented that as males become committed their testosterone levels decrease. The opposite of this is also true - males that leave a relationship see a return to pre-relationship testosterone levels.  This begins to demonstrate early on in the role of testosterone in behaviors.  When males become fathers there are changes in testosterone levels.  Lower testosterones levels are said to play a role in paternal investment and commitment.  It is fair to say that other hormones are at play in paternal investment as well.

Vasopressin.

Vasopressin promotes brain reorganization toward paternal behaviors when the male is cohabiting with the pregnant mother (Palmer, 2002).  This behavior allows other hormones the opportunity to be involved in bonding through this coordinated effort between mother, father, and child.  This occurs through social synchrony, which is the biosocial, coordination of behavior between interacting partners during social contact (Apter-Levi, Zagoory-Sharon, & Feldman, 2014).  The two hormones involved in this process are oxytocin and vasopressin.  It is through this bio-behavioral process that the mother and father support the infant’s entry into the world and demonstrate and prepare the child for socialization.

    Studies have demonstrated the instrumental nature of vasopressin in paternal behaviors (Apter-Levi et al., 2014; Atzil, Hendler, Zagoory-Sharon, Winetraub, & Feldman, 2012).  Parents with higher levels of vasopressin engage in stimulatory contact with the infant, which leads to mutual parent-infant engagement.  Through social synchrony and releases of vasopressin, the father becomes more dedicated to his mate and expresses behaviors of protection.  Through the father’s physical proximity and touch, vasopressin begins to activate bonding behaviors between the father and the mother, helps the father recognize his infant, bond with his infant, and makes him want to be part of the family.  The vasopressin hormone promotes not only the social interest of a human being but tempers the more aggressive nature in humans.  By promoting more rational and less temperamental thinking, this hormone induces a sensible paternal role, providing stability as well as vigilance.  While vasopressin plays a significant role in parent-parent and parent-infant bonding, it usually associated with oxytocin levels as well.

Oxytocin.

While the effects of oxytocin on maternal behavior are well-known, the effects of oxytocin on paternal behaviors are not as established.  Research today demonstrates there is connection between paternal behaviors and oxytocin.  When the father spends significant amounts of time in contact with his infant, oxytocin encourages him to become more involved in the ongoing care.  For example, when an oxytocin nasal spray was administered, they observed that oxytocin modulates parental proximity to infant and movement acceleration.  While parental proximity was associated with maternal behavior and oxytocin, it also was associated with paternal behavior.  Movement acceleration was the most common paternal behavior associated with oxytocin.  Acceleration seemed to capture infants’ attention, which was monitored by longer tracking time.   

    Displaying affection is another behavior necessary in raising healthy infants.   Oxytocin levels in fathers were found to be positively related to the amount of affection a father displayed toward his infant (Feldman, Gordon, Schneiderman, Weisman, & Zagoory-Sharon, 2010).  Those fathers exhibiting both more stimulatory contact and displays of affection with their infant were associated with an oxytocin increase.  While similar oxytocin levels were reported in mother and fathers during parent-infant interactions, fathers’ oxytocin levels were uniquely associated with stimulatory play.  Sometimes it may be discouraged for fathers to engage in “rough” or stimulatory play with their child, but this study proves the opposite - oxytocin is creating and supporting this behavior.  However, it is important to distinguish what appropriate stimulatory play looks like with kids.  While providing just beneficial stimulatory behavior is not sufficient, it is one useful behavior for fathers to form attachments. 

    The role of oxytocin is not unique as it has been associated with vasopressin.  One study demonstrated this relationship when studying male prairie voles — commonly used since they are biparentel mammals. They castrated the males to reduce vasopressin production, believing this would reduce or eliminate paternal behaviors.  They found it did not reduce paternal behaviors (Bales, Maninger, & Hinde, 2012).  This suggests some independence of paternal behavior from vasopressin.  One suggestion is that male prairie voles are able to use either vasopressin or oxytocin to facilitate parenting behavior.  This suggestion was confirmed in follow-up studies, when paternal behavior was not significantly reduced when blocking just vasopressin (Bales, Kim, Lewis-Reese, & Carter, 2004).  Blocking of both vasopressin and oxytocin resulted in elimination of paternal behavior.  Studies like this have demonstrated the shared effect oxytocin and vasopressin have on paternal behavior.  Human studies cannot control hormone outputs such as this, nor can this explain away the complexities of other influences on paternal behavior.  This is useful to the extent of explaining the relationship of oxytocin and paternal behavior.

Opioids.

Even though the research on paternal behavior hormones is less developed, the research specifically relating to paternal behaviors and opioids is even scarcer.  Opioids are typically referred to as the pleasure hormones and are natural morphine-like chemicals created in our bodies (Kovács, Sarnyai, & Szabó, 1998).  They reduce pain awareness and create feelings of elation.  Opioids are also significant in attachment, especially the father-infant bond. Inagaki, Irwin, and Eisenberger (2015) manipulated opioids to examine the relationship between opioids and social bonding behaviors of fathers with infants.  When participants’ opioids were blocked, their feelings of warmth and social connection to infants decreased.  This connection between warm feelings and social connectedness is strongly associated to infant attachment and social attachments.  While these sorts of results on opioids and paternal behavior have implications for father-infant engagement and attachment, the social connected implication is strongest.  Adler understood that social connectedness is key to mental health, and opioids play a role in this social connection. 

    Furthermore, understanding the role opioids in father-infant attachment will be useful in suggestions paternal supportive services.  Opioids are activated by social contact, particularly physical touch induces opioid release, creating good feelings that will enhance bonding.  For example, opioid is released in a child's brain as a conditioned response to a parent’s hug or touch and can be effective for helping reduce the pain from a tumble or a disappointment (Palmer, 2002).  Tolerance to natural opioids can occur, which may interrupt this process of connection in various activities.  When accompanied by higher levels of oxytocin, especially when created through frequent or prolonged body contact, oxytocin will actually inhibit opioid tolerance, protecting the rewards for maintaining close family relationships (Kovács et al., 1998). The relationship between opioids and oxytocin is notable in recommending oxytocin-focus behaviors to fathers. Overall, the significance of opioids is found in the role it plays in both father-infant attachment and social connectedness.

Pheromones

While the directive-nature of vasopressin in fathers has been discussed, pheromones play a role in its ability to initiate hormonal changes in others when detected.  The impact pheromones have on paternal behavior is arguable at best and smaller than imagined.  Pheromones are steroid hormones that are made in the skin (Pheromones, 2015).  It is believed that bodies are somewhat instinctually programmed to react accordingly when it detects pheromones.  The way pheromones work is similar in the way that hormones in the body send specific chemical signals from one set of cells to another, causing them to perform an action.  While pheromones are found throughout all sorts of animal species around the world with different effect sizes, the complexity of human behavior in relation to this hormone must be recognized. 

    Research on the effects of pheromones and humans is still unclear and sometimes largely contradicting; therefore, it is not beneficial to look at human pheromones the same way animal pheromones are viewed.  It is most useful to remember that the main purpose of pheromones has been to communicate.  Humans have developed more complex form of communication than animals, and these play a greater role in decisions.  It is more accurate to say that pheromones cause a physiological response and are one message among many other forms of communication in the body including behavior (Motluk, 2000).  In conclusion, as the effects of pheromones on humans is subtle but nonetheless plays a role that is directive.

Prolactin.

Prolactin has become a well-studied hormone, like oxytocin, that has strong effects on women, men, and infants.  While it has been shown to be critical in milk production in mothers, it is also critical in the initiation and maintenance of maternal behavior (Bales et al., 2012).  A growing body of research has begun to focus on the roles prolactin plays in paternal behavior.  During pregnancy, expectant fathers experience an increase in prolactin in the two weeks prior to infant birth (Storey et al., 2000).  The increase of this is the body’s preparatory response to becoming a father as bonding begins when the baby is in the womb.  During the postpartum period, fathers with higher prolactin levels were more alert and more positive in response to the cries.  Fathers hearing the cries showed a greater percentage increase in prolactin levels compared to first-time fathers or to any group of fathers hearing control stimuli.  Also, testosterone levels decrease around the same time; therefore, they contribute to this effect.