As the Supreme Court considers whether to redefine marriage in genderless terms, scholars supporting gender-diverse parenting filed an amicus brief urging the Court not to eviscerate this fundamental norm of marriage given its crucial benefits to the development of children. If same-sex marriage is constitutionalized, the message the law will send is that the gender of parents becomes valueless, since any two adults will do.

Gender Diversity Is in Our Genes

In the late 1970s, Azim Surani tried to create new life using two sets of genes from only a mother, or a father. Everything then known about genetics suggested that with the right number of chromosomes, life would develop normally, even if all of its genetic material came only from a female or a male. But the eggs with only the mother’s genes could not survive. A similar fate met the eggs implanted with two sets of father’s genes.

As science reporter Paul Raeburn describes, Surani discovered that mothers and fathers each contributed something in their genes that was critical to sustaining life. These “paternal” and “maternal” genes appeared completely indistinguishable in every way, yet expressed themselves differently depending on whether they came from the mother or the father. And both were essential to the survival of the egg.

The essential need for both a mother and a father to provide genetic material for survival parallels what social science tells us about the importance of mothers and fathers in children’s development. Fathers and mothers bring similar, even indistinguishable, capacities that enable healthy child development. But like the complementarity of the left and right halves of the brain, they also bring distinct capacities that provide complementary, irreplaceable contributions to children’s healthy development.

Coo and Cuddle vs. Tickle and Toss

Consider what social science research reveals about how mothers and fathers distinctively influence children’s social and emotional development. Mothers are biologically primed to provide nurturing oriented toward creating a strong attachment relationship. Dramatic increases in oxytocin and oxytocin receptors during the process of giving birth and caring for infants act like a switch in mothers, turning on maternal behaviors. New moms find themselves expressing positive feelings, affectionately touching and gazing at their infants, and engaging in “motherese” vocalizations. Infants’ levels of oxytocin parallel their mothers’, producing feelings of calm and well-being that similarly bond mother and offspring.

Fathers also experience significant physiological changes that “prime” them for bonding. But the same hormones elicit different types of responses. Instead of inviting “security-inducing” behaviors, fathers’ levels of oxytocin are associated with “stimulatory” behaviors, like tickling and bouncing. This suggests a biological foundation for what we observe all around us. While mothers are more likely to “coo and cuddle” their infants, fathers are more likely to “tickle and toss.” These differences foreshadow more extensive complementary patterns exhibited across children’s development.

Identity and Emotional Capacity vs. Social and Relational Capacity

A mother’s capacities are uniquely oriented toward identity formation and emotional security. Her ability to detect, interpret and respond in positive, non-intrusive ways to her infant’s needs has been identified as the strongest and most consistent predictor of a child’s social, emotional, and cognitive development. Neuropsychological studies indicate that mothers have a uniquely sensitive ability to modify the stimulation they give to their infants, matching their infants’ inner state and providing the optimal “chunked bits” of positive interaction needed for development. In the process, children experience positive effects on memory, cognition, stress tolerance, and emotional and behavioral regulation, as well as cardiovascular, metabolic, and immune function.

In this secure attachment relationship, children develop their own sense of identity while learning to appreciate, understand, and empathize with the feelings of others. From infancy on, children are more likely to seek out their mothers for comfort in times of stress. And mothers are much more likely to identify, ask about, listen to, and discuss emotions with children. A mother’s unique orientation toward identifying, expressing, regulating, understanding, and processing emotions is not only important for self-awareness and emotional well-being; it also lays a foundation for moral awareness, including a sense of moral conscience with the capacity to distinguish between right and wrong.

Fathers demonstrate a complementary influence. While mothers are uniquely important in developing secure identity and emotional understanding, fathers are uniquely important in developing social and relational capacity. Interestingly, this complementarity is reflected in the way mothers and fathers hold their infants. While a mother is likely to hold her infant to enable maximum contact with her face and body, a father is most likely to hold the infant in a way that gives the baby the same view of the world as the father has. This “football hold” orients the infant’s face outward, toward others.

It is fathers’ involvement with their children that consistently predicts how they relate to others. Father closeness during a child’s adolescence has been identified as the key predictor of empathy in adulthood, as well as marital relationship quality and extra-marital relationship quality in adulthood. In contrast, lack of father involvement has repeatedly been associated with delinquent and criminal behaviors, even into adulthood. For boys, the mere presence of a father in the home predicts less delinquent behavior.

Some of this may be due, in part to the discipline style of fathers. Fathers intervene to discipline less often than mothers, but when they do, they exhibit more firmness and predictability. In contrast, mothers use more reasoning and flexibility in carrying out consequences. Children, in turn, are more likely to comply with their father’s requests and demands than with their mother’s.

More significantly, fathers influence children’s social and relational capacity through their play. Compared to mothers, fathers are much more likely to interact through play. And that play is strongly predictive of the quality of children’s peer relationships. In repeated studies, fathers who spent more time in positive play with their children had children with the highest peer ratings. When fathers were more responsive, patient, playful and less coercive in their play, children showed less aggressiveness and more peer competence, and they were better liked.

As one report noted, “Rough-housing with dad” appears to “teach children how to deal with aggressive impulses and physical contact without losing control of their emotions.” Through play, fathers help children learn how to temper and channel emotions in a positive, interactive way and gain confidence in their ability to do so. As children age, fathers focus less on physical play and engage in more peer-like verbal play in the form of sarcasm and humor. Peer-like verbal play allows a father to tease and joke with a child, within the safety of the father-child relationship, thus strengthening children’s sense of identity and social confidence. While mothers consistently build self-understanding, fathers consistently build social-relational understanding.

Learning Foundation vs. Orientation and Achievement

This same type of complementarity is exhibited in the mothers’ and fathers’ influence on children’s cognitive development and educational achievement. Indeed, mothers seem to be biologically and psychologically primed to provide just the right kind of emotionally sensitive, cognitively stimulating interactions. Mothers are also are more likely to focus on teaching children in their interactions. For example, while fathers may use a toy to engage with a child, mothers will focus their child on the toy, describe it, and teach about it. This verbally rich, teaching orientation has important implications for cognitive development, including memory, problem-solving, and language advancement.

Fathers complement the foundational contributions mothers make to children’s cognitive development and build upon it. When fathers are “involved, nurturing, and playful,” children exhibit higher IQs, language development, and cognitive skills. One explanation for this is that children with involved fathers show a social-emotional readiness for learning, including being better able to handle the stresses and frustrations associated with schooling. Fathers also uniquely influence children’s expressive language development, because they are more likely to use a broader vocabulary. Mothers, by contrast, often simplify their language to ensure understanding.

Fathers also play a central role in academic achievement. Children with involved fathers were 43 percent more likely to earn “A” grades, 33 percent less likely to repeat a grade, and 98 percent more likely to graduate from college. Part of this is due to the fact that involved fathers are likely to help with homework and provide financial support for college. But involved fathers also monitor and guide children’s behaviors, helping them avoid behaviors that might negatively impact school achievement. Indeed, they seem to be uniquely able to foster a learning environment with just the right mix of “engagement, affection, and supervision.”

Most significantly, fathers build children’s learning capacity by orienting children toward learning in critical ways. First, compared to mothers, fathers’ interactions are characterized by arousal, excitement, and unpredictability in a way that stimulates openness to the world, with an eagerness to explore and discover. Second, fathers have a unique ability to encourage risk-taking while ensuring safety and security, thus inviting children to pursue opportunities that translate into educational and occupational success. Third, involved fathers consistently focus on helping children learn to do things independently and to find solutions to their own problems, building both capacity and confidence. Finally, fathers tend to be more “cognitively demanding” of their children, pushing them to deepen and demonstrate their understanding. Where mothers are more likely to reach in and help children solve a problem, fathers hold back while still offering support, again building capacity and confidence. This area again demonstrates a complementarity between mothers and fathers that is critical, each being irreplaceable by the other.

Gender Identity and Sexual Development

The importance of mothers’ and fathers’ complementarity becomes particularly obvious as a child attempts to make sense of his or her gender. During this period (beginning around eighteen months), both the “same-sex-as-me parent” and the “opposite-sex-from-me-parent” play vital roles. In the words of famed anthropologist Margaret Mead, “One of the most important learnings for every human child is how to be a full member of its own sex and at the same time fully relate to the opposite sex. This is not an easy learning; it requires the continuing presence of a father and a mother.”

The continuing presence of fathers emerges as particularly important in the sexual identity of girls. Girls who are not reared by their biological fathers are much more likely to engage in sexual relations at an early age and to become pregnant as teenagers. Father absence has been identified as the single greatest risk factor in teen pregnancy for girls. In fact, the presence and emotional closeness of fathers seems to “set the reproductive strategy” girls use throughout their lives. Perhaps, as Professor Bradford Wilcox concludes, this is because “Girls raised in homes with their fathers are more likely to receive the attention, affection, and modeling that they need . . . to rebuff young men who do not have their best interests at heart.”

For boys, the effects on sexual development are just as significant but manifest themselves differently. Without the closeness and modeling of a father, boys engage in what David Popenoe calls “compensatory masculinity,” exhibited in rejecting and denigrating anything feminine while seeking to prove masculinity through violent and aggressive domination. In contrast, boys raised in homes with fathers are more likely to “acquire the sense of self-worth and self-control that allows them to steer clear of delinquent peers and trouble with the law,” including in their sexual behaviors.

The Precision of Parental Complementarity

In each of these developmental areas, the natural complementarity between mothers’ and fathers’ parenting strengths is surprisingly precise. Whereas mothers are biologically prepared to nurture, teach, and provide care that is especially important for foundational development, fathers are predisposed to take a facilitative approach to parenting, fostering self-reliance, achievement, and healthy peer relationships in ways that are particularly important especially as children begin to transition to adult life. Both mothers and fathers are needed to create life, and both are needed to best facilitate the nurturing of that life. Mothers do not father, and fathers do not mother. Each emerges as a unique source of distinct and critical nurturing in the development of children. Indeed, evidence of these distinct contributions confirms a long assumed proposition: namely, that the direct, continual involvement of both a mother and a father in the home is ideal for the child’s development.

Thus, mandating that gender diversity be legally removed from marriage destroys one of the most fundamental and scientifically documented norms of marriage, which plays a crucial role in a child’s social, physical, emotional, and intellectual development.

Without gender complementarity in marriage, children will be worse off—and society will suffer the consequences.