3.1. Testosterone

Testosterone has been associated with changes in social information processing, reward sensitivity, and sensation seeking during puberty. Often thought of in relation to aggression, testosterone has also been described as a social hormone, motivating status-seeking and status-maintenance behaviors through a number of diverse mechanisms, including changes in fear processing, stress response, threat vigilance, and reward from increasing social status (Eisenegger and Naef, 2011). Pubertal increases in testosterone have been associated with changes in neural activation to threat cues in the amygdala (associated with threat avoidance) and the nucleus accumbens (associated with reward processing) (Spielberg et al., 2014). In addition, increased testosterone has been associated with increased risk-taking on laboratory tasks among both boys and girls (Op de Macks et al., 2011; Peper and Dahl, 2013). The nucleus accumbens and amygdala, which are social information-processing brain networks that are extensively reorganized during puberty, have large populations of testosterone receptors (Nelson et al., 2005). These testosterone-related processes all likely influence changes in romantic and sexual behavior during adolescence. Consistent with the idea that adolescence is a sensitive period for the behavioral effects of gonadal hormones, animal models suggest that the behavioral response to gonadal hormones differs between pre- and post-pubertal animals. Unlike the pre-pubertal brain, the post-pubertal brain is primed for steroid hormones to activate reproductive behavior (Sisk and Zehr, 2005).

There has been extensive research into the neurodevelopmental trajectories of sexual and mating behavior in male mammals. For instance, in male Syrian hamsters, it is well-established how pubertal hormones influence the structure and function of neural circuits that integrate steroidal and sensory information, and how these changing neural circuits alter the way males respond to social stimuli and engage in mating behaviors (Romeo et al., 2002). Furthermore, research in nonhuman primates has identified that, distinct from most mammalian species, gonadal hormones in primates primarily influence sexual motivation, rather than ability to copulate (Wallen, 2001). Because the effects of gonadal hormones are specific to stimulating sexual motivation, this line of research suggests that shifts in social experience and context are critical influencers of learning about sexual behavior (Wallen, 2001). Nonhuman primate research has also illustrated the importance of the learning component associated with puberty and sexual experience. Although undergoing endocrine puberty and the associated increase in testosterone results in increased sexual behavior, the experience of successful coupling—ejaculating during intercourse with a female—is the best predictor of future sexual behavior independent of elevated testosterone (Wallen, 2001). Even males who have not experienced endocrine puberty increase their sexual behavior following successful sexual experiences. Collectively, the animal research highlights the importance of learning experiences that occur in tandem with the hormonal transition, and suggests new questions for research in humans regarding how hormonal, neurodevelopmental, learning, and contextual changes in adolescence shape the development of sexual behavior and sexual relationships during adolescence.

In humans, whether or not increases in testosterone during puberty directly affect individual differences in sexual motivations and behavior is less clear. Higher testosterone modestly correlates with increased sexual fantasies in pubertal boys, but the effect disappears in models that include onset of spontaneous nocturnal ejaculations and age (Campbell et al., 2005). Increasing testosterone in pre-pubertal boys seems to be associated with increased sexual behavior, including touching others and nocturnal emissions (Finkelstein et al., 1998). In cross-sectional studies, independent of pubertal stage and age, girls and boys with higher levels of testosterone are more likely to engage in sexual intercourse (Halpern et al., 1997, 1998). In contrast, in longitudinal studies, individual changes in testosterone are associated with sexual debut (first sexual intercourse) for girls, but not for boys (Halpern et al., 1997). For boys, pubertal stage is more strongly associated with sexual debut than testosterone (Halpern et al., 1993). These findings highlight the difficulty of disentangling the biologically-mediated effects of gonadal hormones from the socially-mediated effects of bodily changes. For boys, the physical capacity to produce gametes and reproduce occurs relatively early in the pubertal transition, although few boys engage in sexual behavior during that time. Over the course of puberty, testosterone levels increase, boys become taller and more muscular, their voices deepen and their facial hair thickens. These secondary sex characteristics, which are readily apparent and may be seen as attractive or desirable by potential sex partners, may contribute more substantially to a boy’s chances of experiencing sexual intercourse than reproductive capacity or the neurodevelopmental changes in motivation associated with testosterone per se (Halpern et al., 1993). Thus, despite the fact that increases in testosterone are associated with increases in socially-motivated, goal-oriented behavior, they do not automatically translate into increased sexual behavior or activity.

Results from studies that have attempted to unpack the relationship between testosterone and sexual behavior in adults further complicate the picture. For example, studies with young adult men have found that being in a committed romantic relationship is associated with more frequent sexual intercourse and simultaneous declines in testosterone (Burnham et al., 2003; Gray and Campbell, 2009). Among adult women, endogenous testosterone does not show significant correlations with sexual behavior (Roney and Simmons, 2013), but exogenous testosterone treatment in women has been found to increase sexual desire, sexual activity, and sexual self-image (Buster et al., 2005; Davis et al., 2006; Shifren et al., 2006). Both these data suggest that the association between testosterone and sexual behavior may depend heavily on developmental stage, as well as on relationship context.

What we know about testosterone and sexual behavior collectively suggests that there may be some threshold effect for testosterone that leads to the possibility of engaging in sexual intercourse, but no linear relationship between testosterone and subsequent sexual experience. Given the developmental transitions in neural regions with high volumes of testosterone receptors and the increases in testosterone during puberty, we still have much to learn about the relationship between testosterone and romantic and sexual behavior in humans. More nuanced exploration of the relationship between testosterone, sensation seeking, pubertal development and factors associated with learning about sexual behavior, would help clarify the contributions of the biological versus social components that contribute to sexual debut and subsequent sexual activity. In particular, longitudinal studies that can delineate changes in secondary sex characteristics from changes in gonadal hormones can be helpful in identifying specific hormonal mechanisms (Harden, Kretsch, Moore, & Mendle, 2014).

3.2. Estradiol

In addition to testosterone, increases in estradiol and progesterone contribute to remodeling and activation of neural circuitry for both males and females during puberty. Both estradiol and progesterone have been found to play important roles in sexual, social, and risk-taking behavior (Romeo 2003; Tackett et al., 2015; Vermeersch et al., 2009). Compared to other hormones, among girls, estradiol has the strongest correlation to breast development, a very early sign of puberty (Drife, 1986). In contrast to boys (and all other primates) who begin producing gametes before looking sexual mature, human girls develop secondary sex characteristics well before achieving full reproductive capacity. This results in pubertal girls being socially perceived as sexually attractive and desirable before they are reproductively mature or experiencing the neurodevelopmental changes in motivation associated with testosterone. The evolutionary purpose of this divergence is not well understood, but point to the importance of understanding the role of estradiol and progesterone in female sexual behavior.

Limited research has explored the relationship between female hormones in pubertal brain development, sexual behavior, and risk-taking. Among pubertal girls, increased estradiol levels have been associated with increased white matter growth and decreased pubertal grey matter pruning (Herting et al., 2014). Limited functional neuroimaging research has associated estrogen with risk taking in adolescent girls (Vermeersch et al., 2008). The majority of neurodevelopmental studies in humans have identified testosterone, rather than estradiol, as the hormone most correlated with striatal activity related to risk-taking and social motivation in females (Op de Macks et al., 2011; Peper and Dahl, 2013; Peters et al., 2015). Primate studies, on the other hand, suggest that estrogen and progesterone, rather than testosterone, are associated with changes in female sexual behavior, and that the relationship between hormones and behavior is dependent on social context (Wallen, 2001). Tests of this model in humans, also suggest that estradiol is associated with increases in sexual desire and progesterone is associated with declines (Wallen, 2001). In addition, exposing pre-pubertal girls to increased estrogen results in increased kissing and necking behavior (Finkelstein et al., 1998). Although the overwhelming majority of girls choose not to engage in sexual behavior during puberty, understanding the brain, hormone, behavior transitions occurring during this sensitive window may increase our understanding of factors that lead to different behavioral outcomes.

4.1. Parents

Parents play a key role in providing support and information about romantic and sexual behavior. The quality of parent-child relationships affects both sexual behavior and neural development and activation, specifically in the amygdala, which has been associated with reward processing (Ernst et al., 2005), emotional processing (Whalen et al., 2013), and fear response (LeDoux, 2003). The quality of maternal relationships in early adolescence has also been associated with changes in the trajectory of brain maturation. Specifically, more positive relationships between mothers and early adolescents have been associated with decreased volumetric growth of the amygdala (Whittle et al., 2014). These results suggest that maternal relationships may influence brain development trajectories associated with behavior regulation.

The functional role that parental presence has on brain activation also shifts in adolescence. Both children and adolescents have high amygdala reactivity to their mothers’ faces, but amygdala reactivity to the faces of strangers declines from childhood to adolescence. These results suggest that, while positive maternal response remains constant, fear of, and anxiety towards strangers declines across development, facilitating more social exploration (Tottenham et al., 2012). Research has also highlighted that maternal presence can buffer cortisol stress response in children, but it does not have the same buffering effect for adolescents (Hostinar et al., 2014). This suggests that mothers serve to buffer stress for younger children, and as children follow the normal trajectory of development into adolescence, accompanied by declining fear and anxiety about novel people and situations and increased exploration, the physiological effect of parental presence shifts.

Complementing neuroscience research on parent-child relationships, behavioral research has found that positive parent-adolescent relationships are associated with decreased intention to have sex and later age at first sexual intercourse (Van de Bongardt et al., 2014), and adolescents who perceive their parents to be caring have delayed first sex (Longmore et al., 2009). Youth who report having positive relationships and open communication with their parents also have less sexual experience, increased condom use (Parkes et al., 2011), later sexual debut (Price and Hyde, 2008), fewer unintended teen pregnancies (Miller et al., 2001) and fewer sexual partners (Kan et al., 2010; Kerpelman et al., 2016).

Despite the fact that the transition into adolescence is usually accompanied by more autonomy and less parental monitoring, this neuroscience and behavioral research highlights the need for parents not to pull back from parenting during adolescence, but instead shift from providing basic emotional support to providing more constructive support and scaffolding to facilitate the next stage of development. Unfortunately, few resources exist to support parents into caring for adolescents and even fewer resources prepare them for the transition into young adulthood.

4.2. Peers

Peers have also been demonstrated to influence adolescents’ decisions about sexual behavior (Choukas-Bradley et al., 2014; Hampton et al., 2005; Suleiman and Deardorff, 2015). Some research has demonstrated that the presence of peers, or even the suggested presence of peers, increases activation of adolescents’ neural reward circuitry, specifically the ventral striatum (VS), and risk-taking behavior in a way that does not occur for children or adults (Chein et al., 2011; Telzer et al., 2014). One interpretation of this could suggest that adolescents are uniquely vulnerable to take risks in the presence of peers, however, understanding the relationship between increased VS activation and risk taking behavior is far from straight forward. Some studies have found increased VS activation to be associated with increased risk taking on laboratory tasks in the presence of peers (Chein et al., 2011), while other studies have not replicated this result (Peake et al., 2013). Instead these studies have found activation in the temporal parietal junction, an area of the brain involved in self-other metalizing, to mediate the relationship between adolescents’ increased risk taking and their ability to resist peer influence, specifically after experiencing social exclusion (Peake et al., 2013). To further complicate things, other studies have found that increased VS activation during the processing of emotional faces, specifically happy and sad expressions, have been associated with increased self-reported resistance to peer influence (Pfeifer et al., 2011). Collectively, this research suggests that the affective and social context of peer presence may lead to differences in neural activation and to differences in behavior. The transitions occurring in social and affective neural processing systems are priming young people to be open to and enjoy the novel social experiences associated with romantic and sexual relationships. Both platonic and romantic peers influence adolescent sexual behavior and adolescents’ decisions to engage in romantic and sexual relationships (Ali and Dwyer, 2011; Baumgartner et al., 2011; Crockett et al., 2006; Kennett et al., 2012; Potard et al., 2008). In addition, sexual activity is strongly peer-mediated; having sexually active platonic peers is associated with earlier first sexual intercourse, more frequent sexual activity, and more sexual partners (Ali and Dwyer, 2011; Furman et al., 2007; Santor et al., 2000).

4.3. Media

In addition to social relationships “in real life,” traditional media and social media may also shape sexual behavior. Messages about sexuality and romantic relationships are persistent throughout the media. Over 70% of television programing contains some type of sexual content or dialogue (Kunkel et al., 2005). Depending on the context and population, between 23 and 95% of 10–19-year-olds report having ever viewed online pornography, and between 28 and 84% reported that the exposure was unwanted or unintentional (Peter and Valkenburg, 2016; Wolak et al., 2007). Mass media has been identified as a “sexual super peer” having significant social influence, particularly for girls who experience puberty earlier than their peers (Brown et al., 2005).

Despite this significant exposure to sexual content and messages throughout adolescence, little is known about the effect of sexual content on brain development. More generally, media content has been found to impact neural functioning. For example, limited exposure to violent media among adolescents has been found to affect the developmental trajectories of frontal inhibitory systems and subcortical limbic structures, as well as the connectivity between them, and may have some effect on violent behavior (Hummer 2015; Kalnin et al., 2011). Although we are not aware of any imaging research that has been conducted specifically on sexual media, it is highly probable that sexual and romantic images, abundant in everything from general media to pornography, also likely affect neural development and behavior. At the same time, individual differences in neurodevelopment may shape exposure to sexual media. For example, more advanced pubertal young men with high sensation seeking tendencies are most likely to seek out online pornography intentionally, and there is a strong association between increased pornography exposure and less restrictive sexual attitudes (Peter and Valkenburg, 2016).

Developmental science offers strategies to expand our understanding of the impact of the media on neural development and sexual behavior. Developmental adaptations of innovative communications neuroscience research (e.g. Falk et al., 2015, 2012) coupled with behavioral research could help us better understand the neural activation associated with adolescents viewing different types of romantic and sexual media and better understand the types of affective learning experiences resulting from these media contexts. Applying a developmental framework to communications neuroscience could help inform the development of positive romantic and sexual media messages and increase our understanding of the potential negative trajectories associated with viewing more harmful messages. In the age of increased access to a wide range of romantic and sexual media content and the rapid emergence of virtual reality pornography, the need for these insights is urgent. Given that adolescence is a critical period of romantic and sexual identity development, we need a better understanding of the relationship between neurodevelopment, exposure to the romantic and sexual media, and subsequent behavioral trajectories.

5.1. Example 1: extended adolescence

Among humans, adolescence is a socially constructed period beginning with the hormonal, psychological, and physical changes occurring with puberty. The end of adolescence is much less clearly defined. Many of the features of adolescence are socially constructed, but, across species, it is observed as a “time of preparation for adult reproductive life,” spanning the period between achieving physical readiness to engage in sexual activity and having social permission to reproduce (Schlegel 1995, p. 16) . Across primate species, young adolescents engage in mating and sexual behavior but rarely produce offspring (Schlegel, 1995). In humans, the length and experiences associated with adolescence can vary widely; nevertheless, the period, characterized by increasing autonomy, but free of the full array adult responsibilities, is culturally universal (Schlegel, 1995).

Today’s youth face an especially protracted period between when they are physiologically capable of reproduction and when reproduction is socially and personally desirable. In countries across the globe, the age of puberty has continued to decline (Parent et al., 2003; Sørensen et al., 2012). For girls, this has been measured primarily by decline in menarcheal age. As menarche occurs rather late in the pubertal cascade, this metric may underestimate the extent of historical declines in the average age at pubertal onset. The average age of the most common early physical sign of puberty among girls, breast budding, has declined more rapidly than age at menarche, and as a result, girls are spending longer amounts of time in the pubertal transition (Mendle, 2014). Interestingly, the decrease in age of onset of breast development has not been clearly associated with early increases in the sex hormones associated with puberty, including gonadotropins and estrogen (Sørensen et al., 2012). In contrast, declines in the early physical sign of puberty for boys—primarily testicular growth— have been paralleled by secular shifts in hormones associated with puberty (Sørensen et al., 2012). The difference in the developmental trends between boys and girls is not well understood, but it highlights the importance of understanding the impact of these different trends on pubertal neural development, both within and between sexes. For both sexes, there is a clear declining trend in the age when humans are biologically capable of reproduction. Given that some brain development trajectories are closely tied to changes in pubertal hormones, it is likely (although an outstanding empirical question) that neural developmental changes associated with puberty have also begun to trend earlier.

At the same time that the age of reproductive capacity has declined, the age at which it is socially desirable for young people to bear children has continued to increase. The age at which young women in higher resourced countries first bear children has significantly increased over the past 40 years, and this trend is now beginning in many lower and mid-resourced countries as well (Bearinger et al., 2007; Bongaarts and Blanc, 2015; Mathews and Hamilton, 2009; Sedgh et al., 2015; Westoff 2003). Today, across the globe, the mean age of women at their first birth ranges from 20.9 years old in sub-Saharan Africa, to 25 years old in the US (Bongaarts and Blanc, 2015; Mathews and Hamilton, 2009).

Until recently, social acceptance of reproduction has been highly correlated with marriage, and the first child was often born within the first few years of marriage. This trend has also experienced disruption in recent years. First, even more sharply than the age of first birth, the age of marriage—highly correlated with socioeconomic and demographic variables—has increased for both men and women across the globe (Westoff, 2003). Second, particularly in higher resourced countries, there has been a trend towards uncoupling marriage and childbearing, with more births occurring outside of legal marriages, fewer marriages as a result of premarital conception, and married couples waiting longer after marriage for childbearing (England et al., 2013; Hayford et al., 2014). There is also an increasing number of people who go through the biological transition of puberty but never engage in marriage or parenting, and yet still have fulfilling romantic and sexual lives. Together these trends highlight the importance of gaining a better understanding about the intersection of social, cultural, and biological factors that influence normative trajectories of romantic and sexual development. This points to a wide range of yet unexplored empirical questions about how physical and neural development interact with contextual factors to shape and influence romantic and sexual behavior.

The secular trends of early puberty and later childbearing provide great opportunities for young people to have a longer period for increased maturation, educational attainment, and stabilization prior to assuming the full scope of adult roles and responsibilities. At the same time, many individuals now spend a decade or more of their lives feeling biologically, physiologically, and motivationally primed to engage in romantic and sexual relationships outside of the context of reproduction. As such, we need to better understand the behavioral outcomes and neural developmental underpinnings of the motivations associated with early romantic love and sexual experiences, in order to provide young people supports and scaffolding that facilitate positive trajectories.

5.2. Example 2: contraception innovation

Over the course of the teen years, the majority of young people become sexually active. Globally, the average age of sexual debut ranges from 16.5 to 24.5 years old for males and from 15.5 to 21.5 years old for females (Wellings et al., 2006). In light of the fact that many young people engage in sexual intercourse before they want to parent, many young people choose contraception. Although condoms, cervical caps, diaphragms, and some intrauterine devices (IUDs) offer non-hormonal contraception, the primary methods of contraception used by adolescents include hormones. The recent global policy push to increase the use of long-acting reversible contraceptives (LARCs) among adolescents has promoted the use of some very effective non-hormonal methods, like hormone free IUDs, and has promoted increased use of IUDs, implants, and injectables containing progestin (Ott et al., 2014). Given the evidence that pubertal hormones influence neurodevelopment, it is important to identify if and when there could be negative implications of disrupting the normal hormonal trajectories, and specifically the potential influence on neural development, during critical developmental windows. To date, we are not aware of any published data exploring these effects.

Despite an array of other options and the promotion of LARCs, combined oral contraceptives (COCs), containing both estrogen and progestin, remain one of the most utilized contraceptive methods among young women (Ott et al., 2014). COCs, which are highly effective for preventing pregnancy when used correctly, have been found to significantly suppress both free and total testosterone levels by up to 50% in adult women (Zimmerman et al., 2013). This suppression of testosterone has been thought to be the underlying source of complaints associated with COCs, including diminished well-being and quality of life, reduced libido, cognitive disruptions, and decreased bone mass (Zimmerman et al., 2013). Although women on COCs have overall lower testosterone levels, additional research has suggested that the change in testosterone when women are engaged in a competitive tasks remains constant despite COC use (Edwards and O’Neal, 2009). Once again, this suggests interesting empirical questions about how COC use could affect testosterone levels in young women who are in the midst of the developmental testosterone increase associated with puberty (Braams et al., 2015).

In addition to the adverse effects reported by adult women, there are additional potential adverse effects of testosterone reduction during adolescence. Given that testosterone has been shown to motivate increased interest in arousing experiences, inhibiting testosterone levels during the peripubertal or adolescent period may diminish motivation to engage in a range of prosocial risk-taking behaviors that support positive developmental trajectories. It is also equally possible that inhibiting testosterone will not limit positive or negative risk taking, for although overall testosterone levels may be suppressed in young women taking COCs, their testosterone responses to stimuli may remain constant. Understanding the impact of testosterone suppression in adolescence on neural development would be essential to help better understand the mechanisms that may contribute to either of these potential trajectories. Furthermore, insights into the implications of chronic suppression of testosterone, in concert with the hormonal orchestration of puberty, may also help to bolster innovation as pharmaceutical companies develop new contraceptive methods. This is a place where animal neurodevelopmental models may offer important insights that can be later tested in humans. We do want to state explicitly that our goal is not to introduce new barriers to young women seeking reliable contraception, but instead propose that having more information about the interaction between hormonal contraceptives and neurodevelopmental trajectories would help increase the safety, efficacy and impact of contraceptives for young women.

5.3. Example 3: childbearing and parenting

The global trend towards later childbearing has been a positive trajectory, as there is clear consensus that very early childbearing, before age 15, has detrimental effects on the health, socioeconomic, educational and social outcomes of both mothers and infants (Brooks-Gunn & Furstenberg, 1986; Gibb et al., 2014; Hofferth and Reid, 2001; Brooks-Gunn and Furstenberg, 1986). Despite this knowledge on outcomes, we have very little insight into the neural developmental impacts of early childbearing. For over 100 years, we have understood that pregnancy, childbearing, lactation, and parenting involve a precise sequence of hormonal transitions. Pregnancy involves dramatic increases in human chorionic gonadotropin (HCG), estrogen, and progesterone. Labor follows a continued increase in placental production of estrogen and progesterone coupled with increases in oxytocin, endorphins, and prolactin. In contrast, lactation involves a rapid and severe drop-off in gonadal hormones and increase in prolactin (Russell et al., 2001). Rodent models illustrate that the voluntary, proactive, appetitive aspects of maternal behavior, including seeking out and retrieving pups, are highly linked to the effect of pregnancy hormones stimulating the development and activation of the mesolimbic dopamine system (Numan and Stolzenberg, 2009). This highlights how although primarily associated with rewards, the dopamine response systems are also important for learning about parenting. In an attempt to bridge from animal to human models, Moses-Kolko and colleagues tested, but were unable to find, that motherhood affected ventral striatal response in expectation of a financial reward (Moses-Kolko et al., 2016). The lack of replicability points to the importance of developing ecologically valid paradigms to test hypothesis about the impact of pregnancy on brain development trajectories, as financial reward is distinctly different from the social and emotional reward associated with nurturing an infant. Women of all ages describe a range of physical and cognitive changes associated with the hormonal transitions associated with childbearing, but little is known about how experiencing these hormonal events during the pubertal transition, or closely following it, affects neurodevelopmental trajectories.

Understanding the impact of delayed childbearing is equally as important as understanding the impact of early childbearing. Neural flexibility, defined as the extent to which cognitive control and social and affective motivation systems are engaged and activated under different circumstances, has been found to persist into the third decade of life (Crone and Dahl, 2012). Up until very recently in human history, it is during this time that the majority of individuals engaged in mating, bonding, and parenting activities. The recent trend to delay parenting, often well into the fourth decade of life, presents some intriguing questions about the interactions between biology and experience in influencing neural development. In addition, the increasing separation between the onset of reproductive maturity and first childbearing creates opportunities for both boys and girls to have an extended period when they have expanded autonomy and independence to pursue education and other life goals. It also minimizes the need for early pair-bonding and instead fosters an opportunity for young people to explore romantic and sexual relationships with a number of different people. In higher resourced countries such as the US and Europe, the overwhelming majority of young people have romantic and sexual relationships before they commit to a single partner (Guttmacher Institute, 2014). Moreover, in countries where the span between puberty and childbearing has just begun to elongate, such as sub-Saharan Africa and South America, approximately 25–30% of adolescent girls engage in sex before coupling (Guttmacher Institute and International Planned Parenthood Federation, 2010). Developmental neuroscience may help identify the different neurodevelopmental trajectories associated with young people having multiple dynamic, high intensity romantic relationships with a broad range of people, and young people who have longer, more intense relationships with just a few. It may also identify different neural trajectories among people who engage in parenting during the first three decades of life and those who delay.

In addition to women, men also experience hormonal transitions associated with mating and parenting. In the US, single adult men who are seeking romantic partners and paired men who are seeking relationships outside of their primary partners have higher testosterone levels compared to men in committed relationship and furthermore, men who are fathers, regardless of relationship status, have the lowest testosterone levels (Gray and Campbell, 2009). Interestingly, this relationship trend holds in some countries, but not in others, begging important questions the causal direction and the reciprocal relationship between hormones and context (Gray and Campbell, 2009). Putting a developmental frame on this research about childbearing and parenting suggests that the pace of neural maturation from adolescence to adulthood is not entirely determined by a strict ontogenetic timetable but is rather shaped by the demands of an individual’s social context. As an entire generation forestalls marriage and parenthood, in some cases entirely, it would behoove us to better understand what effect this has on “typical” trajectories of neural development into the third decade of life.