Understanding the Complex Role of Hormones in Social Behavior

Social behavior, in both humans and animals, is not as straightforward as positive (bonding) or negative (aggression) interactions. It is influenced by various factors, such as memory, recognition, and hormones, which play a critical role in determining how individuals interact with each other. Two of the key hormones involved in social behavior are Oxytocin (Oxt) and Vasopressin (Avp), which are often termed as “sister hormones” due to their structural similarity. These hormones, synthesized in the hypothalamus, have been found to significantly affect social recognition, bonding, and even aggression.

Memory and Social Recognition: Friend or Foe?

When deciding how to interact with others, one of the first steps is determining whether the individual is familiar—whether they are a friend or foe. This decision often depends on social recognition memory, which involves recalling past experiences or interactions. Research suggests that Oxytocin and Vasopressin play essential roles in forming these recognition memories, which in turn influence whether we approach or avoid others.

Vasopressin, in particular, has been linked to the formation of long-term recognition memories. A fascinating study involving prairie voles—small mammals known for their monogamous pair-bonding—demonstrated this. When scientists injected these voles with a vasopressin antagonist (a substance that blocks the hormone), their ability to form mate bonds was significantly diminished. This study highlights how critical vasopressin is in social memory, particularly in maintaining long-term relationships.

Aggression: A Social Tool

Aggression often carries a negative connotation, but in the animal kingdom, it plays a crucial role in establishing and maintaining social structures. In species that do not form lasting social bonds, aggression serves as a tool for resource distribution and can actually reduce social conflict. For example, many mammals establish dominance hierarchies through aggressive behavior, and once these relationships are set, social communication—such as scent marking or vocalization—helps to maintain the hierarchy without constant fighting.

Early research on squirrel monkeys revealed how oxytocin and social status intersect to influence aggressive behavior. In these studies, oxytocin administered to dominant males increased aggression, while subordinate males were unaffected. Conversely, oxytocin increased scent-marking behaviors in subordinate males, showing that social experience and status shape how oxytocin affects behavior.

This dynamic can also be seen in rats, where dominant males show higher levels of oxytocin receptor mRNA in the medial amygdala after a social encounter that establishes dominance. This suggests that oxytocin signaling helps maintain these social hierarchies. Blocking oxytocin receptors in subordinate rats, for example, prolongs the dominance of their superiors, further illustrating oxytocin’s role in sustaining social order.

Social Status and Oxytocin

This phenomenon isn’t exclusive to rodents and monkeys. In primates, including rhesus monkeys, dominant individuals—especially females—show significantly higher levels of circulating oxytocin compared to subordinates. This suggests that oxytocin is not just a hormone of bonding but also plays a role in maintaining social dominance, further complicating its role in social behavior.

The Complexity of Human Social Motivation

Human social behavior, like that of animals, is complex and cannot be boiled down to simply bonding or aggression. Our social motivations are influenced by a myriad of factors, including basic needs, memory, and external stimuli. For instance, cooperation and friendliness likely evolved from necessities such as raising offspring or securing resources, which over time led to deeper social connections.

Living in social groups offers many advantages, such as better security, efficient food procurement, and improved mating opportunities. However, social living also presents challenges. Close proximity can lead to disease transmission, and within-group aggression must be controlled. This makes memory and recognition crucial for managing social interactions effectively. Recognizing others through memory, for example, helps to maintain group cohesion and minimize conflict.

The Social Behavior Neural Network (SBNN)

The Social Behavior Neural Network (SBNN) hypothesis, proposed by Newman in 1999, provides a more nuanced understanding of how social behaviors are controlled. This hypothesis suggests that a network of interconnected brain regions—rather than a single brain area—works together to regulate social behavior. The SBNN includes areas such as the extended amygdala, bed nucleus of the stria terminalis (BNST), lateral septum (LS), periaqueductal gray (PAG), medial preoptic area (MPOA), and ventromedial hypothalamus (VMH).

These brain regions are connected in a reciprocal manner, contain receptors for gonadal steroid hormones (such as testosterone and estrogen), and are involved in more than one type of social behavior. Instead of focusing on a singular region, the SBNN framework emphasizes the emergent properties of these regions working together to create social behaviors. For example, this network governs not just aggression but also social recognition memory, parental behavior, and social communication.

Motivated Behaviors and Social Decision-Making

Social behavior is also intertwined with motivational systems in the brain, particularly the mesolimbic dopamine (DA) system, which influences how we respond to rewarding or aversive stimuli. The ventral tegmental area (VTA), where dopamine-producing neurons reside, is a key player in this system. These neurons send signals to other areas involved in motivation and emotion, helping to determine the value of different stimuli and prompting the appropriate behavioral response.

Importantly, the SBNN and mesolimbic systems interact, especially when it comes to social decision-making. For example, areas like the lateral septum (LS) and extended amygdala act as relay stations, conveying information from the mesolimbic system to the SBNN about the salience of a social stimulus. This interaction ensures that social behaviors are tailored to the specific context and needs of the individual.

The Role of Oxytocin and Vasopressin in Motivation

Oxytocin and vasopressin are not only crucial in social recognition and bonding but also in influencing the motivational systems that drive behavior. Their receptors are found in key brain areas involved in emotional regulation, motivation, and reward processing, such as the amygdala, hippocampus, ventral tegmental area (VTA), prefrontal cortex (PFC), nucleus accumbens (NAcc), and ventral pallidum (VP). This positioning allows these hormones to modulate behaviors linked to the dopamine system, further intertwining social behavior and motivation.

Conclusion

Social behavior is an intricate blend of biological mechanisms, memory, and external stimuli, shaped by evolutionary pressures and governed by a complex network of brain regions. Hormones like oxytocin and vasopressin play a pivotal role in these processes, influencing not only bonding and aggression but also memory, social status, and motivation. The interaction between different brain networks—such as the SBNN and the mesolimbic dopamine system—highlights the complexity of social decision-making, which cannot be reduced to simple binary outcomes. Understanding these systems offers profound insights into both human and animal social behavior, revealing the sophisticated machinery behind everyday social interactions.

Reference: Caldwell HK, Albers HE. Oxytocin, vasopressin, and the motivational forces that drive social behaviors. Behavioral neuroscience of motivation 2016:51–103.