Research on the biological basis of implicit motives has a number of unanswered questions that remain open for exploration, some of which we would like to highlight. While animal research has demonstrated direct effects of the catecholamines and cortisol on changes in testosterone, this has not been demonstrated directly in humans. Rather, the reviewed studies separately examined situation x n Power effects on each hormonal component of our model. It has yet to be shown within a single study that all components of this model fall into place, which is an important direction for future research. Moreover, such a study would also allow us to examine the possible cross-talk between the HPA and SAM axes in humans.
Despite a clear set of relationships between n Power and testosterone in men, studies have not consistently linked testosterone to n Power in women (Schultheiss, 2007
). Broadly speaking, the majority of behavioral endocrinology research on dominance in humans has focused on testosterone and principally used male subjects (Mazur & Booth, 1998
). However, animal studies have demonstrated that estradiol can positively influence dominance behavior or the motivation to attain dominance in females of several mammalian species (Boissou, 1990; Farruzzi et al., 2005; Michael & Zumpe, 1993
; Zehr et al., 1998
; Zumpe & Michael, 1989
). Some researchers have proposed that estradiol might have a more direct connection to dominance in women (Cashdan, 1995
; Schultheiss, 2007
). In response to these speculations, Stanton and Schultheiss (2007)
examined the relationship between n Power and estradiol in women. Replicating an earlier observation by Schultheiss, Dargel and Rohde (2003), they found that baseline estradiol levels and n Power were positively related. Further exploring the n Power-estradiol relationship, Stanton and Schultheiss (2007)
employed a dominance contest method similar to the one previously used with men (Schultheiss et al., 2005
) to examine estradiol changes after a dominance contest. They found that higher levels of n Power were associated with greater estradiol increases after winning. Conversely, after losing a dominance contest, higher levels of n Power were associated with greater estradiol decreases. While our model of catecholamine and cortisol changes driving testosterone changes works well in explaining the effects of n Power on men’s testosterone responses to winning and losing a contest (Schultheiss, 2007
), the extent to which catecholamine and cortisol responses to dominance challenges can have similar effects on estradiol release in women is unknown. Whereas the research by Sapolsky (1985
) explained the biological precursors to testosterone change in males, research has yet to document biological precursors to rapid estradiol changes in females. A greater focus on female dominance and its biological correlates is needed in both human and animal research.
Replication of the estradiol effects in women as a response to dominance contests is imperative. Further, if estradiol changes also mediate behaviors that are instrumental to the outcome of a dominance contest, that would suggest that estradiol change is not only a response to the situation, but is also critically linked to the shaping of the behaviors are instrumental to the contest outcome. Documenting such behavioral mediation by estradiol in women would make estradiol a more complete parallel to testosterone in men, since testosterone changes mediate such behaviors in men (Schultheiss & Rhode, 2002
; Schultheiss et al., 2005
). Moreover, the subsequent changes in behavior and social cognition as an effect of estradiol change in women are also unknown and would be a potential area for future exploratory research. By placing such experiments in a broader context, exploration of changes in real-life outcome behaviors as a function of testosterone or estradiol change in response to dominance contests would bolster this line of research with greater ecological validity.
Exploration of a potential relationship between frustrated n Power, cortisol, and depression is also a worthy path for research. Frustrated n Power has been linked to immune system impairment, heart disease, and excessive consumption of alcohol (see McClelland, 1987
for a review), and the positive link between n Power frustration and cortisol release could compellingly extend this line of research into explorations of psychopathology including depression.
From a neuropsychological perspective, fMRI holds considerable promise for the examination of the neurological basis of individual differences, and researchers in personality neuroscience are beginning to exploit this tool. Schultheiss and colleagues (2008)
recently published the first study to examine the moderating role of n Power on patterns of brain activation. With relevance to the biological model of n Power, the hypothalamus is largely in control of hormone axes (hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal), as well as aspects of dominance behavior. Studies of dominance could use neuroimaging to measure the relationship between brain activation of the hypothalamus and its connection with other parts of the emotional brain and subsequent hormone release as a function of n Power. Such work could help further uncover how the brain orchestrates the complex hormonal responses to dominance challenges and stressors in the context of implicit power motivation.