An essential characteristic of organisms living in a social hierarchy is the ability to control individual impulses. Inhibitory control makes it possible for a low status individual to defer to a higher status individual when a limited resource is sought by both. Without this arrangement the social structure would quickly collapse into chaos. In the higher primate species the ability to assess the status of other individuals and to modify oneís own behavior accordingly is in large part mediated by the developed frontal cortex. This development of the frontal cortex has of course reached its highest pinnacle in humans who are capable of making very complex assessments of very intricate situations and modifying their behavior in very complex ways to accommodate. Although the frontal cortex is of undoubted importance in complex primates, many animal species are quite capable of maintaining social hierarchies with very little cortical development. A much more primitive physiological mechanism is involved in the regulation of dominance behavior.
The extremely primitive nature of this physiological mechanism is indicated by the fact that it is found in an extremely wide diversity of species from the phyla Annelida, Arthropopa, Mollusca, Platyhelmenthese, and Chordata (Turlejski, 1996). Although this system varies in its particulars from species to species, it is always involved in the modulation of motor behavior, varying its tempo or inhibiting it altogether. The neurons, or nerve cells, in this system use a neurotransmitter called serotonin. Neurotransmitters are chemical messengers released from the ends of nerve cells in order to facilitate the transmission of an electrical signal to the adjacent nerve cell. The remote Metazoan ancestor that we have in common with flatworms, leeches, crayfish, and all sorts of other creatures must have had this type of serotonergic system. In primitive solitary organisms, increases in serotonin activity would lead to increases in motor activity. Conversely, decreases in serotonin activity would lower motor behavior or inhibit completely. If an organism happened upon a rich foraging opportunity the firing rate of its serotonin neurons would increase and as a result the animal would have more energy to take advantage of the abundant resources it had found. If, however, this same animal sensed the presence of a predator there would be a lowering of serotonin activity and a corresponding inhibition of its motor behavior. Immobility is a primary defense against predation as long as the predator has not spotted the prey animal. As animals evolved to live in a hierarchically organized social group, these primitive serotonergic motor systems were co-opted and modified to facilitate the new behavioral demands.
McGuire and Raleigh (1975) demonstrated that serotonin level covaries with changes of status in vervet monkeys. Moreover, artificially raising the serotonin levels in subordinate vervet monkeys with fluoxetine (Prozac) results in these individuals rising in status, in some cases to alpha rank (Raleigh, 1991). Dominant animals display an air of calm self-assurance, self-control, and self-directed behavior. Subordinates, on the other hand, appear fidgety, easily perturbed, and their behavior seems to be largely controlled by external stimuli rather than being self-directed. Subordinates are also prone to impulsive behavior including impulsive aggression. The higher serotonin levels and correspondingly higher levels of motor activity in the dominant animals provides them with the means to take advantage of their relatively better resource opportunities. In the subordinates, lowered serotonin levels are adaptive in that motor activity is lessened thus conserving energy in the face of diminished access to food. Moreover, their behavioral inhibition helps to reduce potential conflict with higher-ranking animals. Low serotonin is also associated with the hyper-vigilant state, which, of course, allows the subordinate to keep track of potentially dangerous higher-ranking animals.
Impulsive behavior is another characteristic associated with lowered serotonin in the low-status individual. A direct association between low serontonin production and impulsive aggression has been demonstrated in numerous species including humans (Roy & Linnoila, 1988; Kalat, 1997). The adaptive significance of impulsivity in the low-ranking individual may not be immediately apparent. Keep in mind, however, that deferment to higher-ranking individuals by lower ranking individuals is adaptive only up to a point. If an individual is extremely low ranking, or if resources are in very limited supply, or these two factors interact at some critical level, then absolute obsequiousness by the low ranking individual can result in zero fitness. Impetuously snatching a morsel of food from a higher-ranking group member or, even more hazardously copulating with a female ďbelongingĒ to a higher-ranking male may raise a low ranking individualís fitness level. Individuals of middle rank improve their fitness via a moderate level of impulsiveness, with their behavior being a mix of socially acceptable patterns and rash actions.
Consistent failure by an individual to control impulses makes it unlikely that the individual can remain integrated in the group. The fissioning of a very low ranking individual from the group may increase the potential fitness of that individual via alternative social strategies (e.g., clandestine copulations, forming or joining different groups).
Research into the etiology of low serotonin levels and impulsivity suggests, that like other Old World primates, individual humans may be predisposed toward this condition by their genetics (Eysenck, 1983; Plomin, 1976). To explain why genes for strong impulsivity and chronically low serotonergic activity are maintained in populations resulting in a small percentage of inherently impulsive individuals, one has to look at selective pressures in the totality of time. High impulsivity generally lowers fitness in an established stable group. However, during times of group upheaval (e.g., internecine conflict or invasion by outsiders) the highly impulsive individuals would have increased fitness relative those group members who continued to behave as though the status quo relations still applied. These periods of group upheaval, though relatively rare, would be sufficient to maintain impulsivity genes despite their reduced fitness during stable periods.
Individual developmental history probably plays a larger role in determining the degree to which impulsive tendencies are manifested. Research by Higley, Suomi and Linnoila (1996) indicates that an impoverished rearing environment (i.e., peer-reared) produces individual rhesus monkeys with chronically low serotonin levels and tendencies toward excessive aggression and impulsivity. In humans the rate of childhood physical abuse and sexual abuse reported from patients with Borderline Personality Disorder (BPD) is above 70% (Ludolph et al., 1990; Ogata et al., 1990). BPD and a number of other disorders associated with compromised impulse control such as eating disorders, obsessionality, and pathological aggression have been shown to be at least in part due to hyposerotonergic activity and consequently amenable to treatment with serotonin reuptake inhibitors (SRIs) (Markovitz, 1995).
The nuclear family is the first social group into which people are fully integrated. Studies by Palmer, McCown, and Kerby (1997) and Palmer, McCown, and Thornburgh (1998) demonstrate that childhood environments high in parental nurturance produce individuals predisposed to high-status functioning as adults (e.g., they are highly sociable, responsible, self-controlled and low in impulsivity). Conversely, childhood environments low in parental nurturance and high in family discord produce individuals predisposed to low-status functioning as adults (e.g., they have low levels of sociality, responsibility, self-control and high levels of impulsivity).
Genetic, ontogenetic, and proximate factors all play a role in determining an individualís level of serotonergic functioning and concomitant behavioral tendencies. Because higher status is associated with greater fitness, we have evolved motivational tendencies that push us toward achieving higher social rank. An improvement in status and a corresponding elevation in serotonergic activity is experienced as an elevation in mood state. Common terms used to refer to a highly elevated mood state include joy, happiness, and euphoria. This subjective state is analogous to the pleasure we feel when eating a sumptuous meal when we are really hungry. We experience pleasure when eating because the pleasure insures that we will continue to engage in the survival-promoting behavior of eating. By the same token, we try to avoid the unpleasant sensations of hunger because it signals a threat to our survival. Mood states function in a similar manner to direct us toward survival-enhancing patterns of behavior.
If an individual suddenly discovers that they possess the winning lottery ticket and that they are now in possession of a huge sum of money, they will immediately experience a surge in serotonergic activity and a flood of stress hormones. Their subjective state will be one of euphoria and they will have extremely high levels of energy. This will be partly due to the serotonin, but also a result of the stress hormones, which they experience as a state of elated excitement. This elevation in energy will enable them to utilize this sudden flood of resources. Whereas before, a simple trip to the local grocery store would have been perceived as tiring the same individual now can shop twenty hours of the day without experiencing any fatigue. Compare this behavior with that of the individual who has just experienced a traumatic loss. For example, they have just been fired from their job. This individual will experience a sudden decrease in serotonergic activity and their system will also be flooded with stress hormones. In the case of this individual, the stress hormones will not cause them to feel elation and excitement, but rather anxiety, apprehension, and nervousness. The low serotonin activity will be subjectively experienced as a state of depression. Once the initial rush of stress hormones has diminished, the individual will experience a general state of depression. They will feel lethargic and possess very low levels of energy. In the ancestral hominids such a depressed state may have had an adaptive function. The individual who found his or herself at the fringe of the group, with very low social rank, would have benefited from conserving energy as much as possible since they would have been last in line for any food resources that came available. They also would have been less likely to challenge higher-ranking individuals in the group and thus would have avoided potentially harmful conflict. This depressive state may no longer have such an adaptive role in the modern world in which we live. A state of profound lethargy may prevent the individual who has just lost a job from actively seeking another job. (Of course, it should be noted that depression has a complex etiology, and the evolutionary explanation is only one part of the etiology.)
Special pharmaceutical drugs, which work by blocking the re-uptake of serotonin, were discovered to have a mood elevating effect on individuals suffering from severe depression. Selective serotonin re-uptake inhibitors (SSRIís) such as the brands Prozac, Zoloft, and Paxil became very popular in therapeutic usage many years before the function of serotonin was well understood. Early popular books about such drugs such as Listening to Prozac (Kramer, 1993) advocated the use of these drugs for people who were shy and felt intimidated in the work place. In light of the evidence for the role of serotonin and status functioning, this advice seems particularly credible. However, analogous to drugs that numb our sensitivity to physical pain and therefore make us more vulnerable to injury, the SSRIís may have hidden dangers. The use of Prozac and similar drugs may induce a subjective state that mimics that of a high-status individual. Being highly self-directed and fairly oblivious to other people may work well for a truly high ranking individual, but for a low ranking individual to feel and act this way may prove problematic if not disastrous. For example, the SSRI-using employee that begins to habitually ignore their boss and to modify their work activities according to their own whims may find themselves among the unemployed.
Psychiatrist Russell Gardner described a patient of his who was able to advance her career presumably because of her treatment regimen of Prozac (1998). While this is the case with some individuals, Gardner pointed out that human beings as a whole do not rise in status automatically as a result of using SSRIís. The vervet monkeys studied by Raleigh and McGuire did rise in social rank as a result of this type of pharmaceutical intervention, but in humans the interaction appears to be more complex. A host of biochemical factors including numerous neurotransmitters, reproductive hormones, and stress hormones are all effected by changes in social status. Of these biochemical factors, serotonin is probably the one that is most central in its relation to status functioning. However, it seems doubtful that even in great apes could the manipulation of serotonin effect dominant status. In chimpanzees, for example, the cognitive complexities are already on a scale that probably precludes simple manipulation of dominance rank through a single neurotransmitter.