Biomarkers of Human Experience
Early attempts to use biomarkers to understand some of the“invisible”aspects of
other people come from endocrinology. The effects of hormones on behavior have
been recognized for a long time, with some reference to this relationship in
Aristotle’s writings (Leshner 1978 ). Much of this early work focused on the effects
of hormones on reproduction and related behavior in animals, particularly birds
(Beach 1975 ). Hormones have fairly direct effects on behavior through their
influence on the nervous system, as well as on general metabolism. On the other
hand, behavior leads to encounters with various stimuli that cause secretion of
specific hormones (Whalan 1967 ). Thus, the correlation between hormones and
behavior represents a form of reciprocal causality.
The relation between hormones and human behavior is particularly evident in
studies of reproduction. As noted, life history theory suggests that individuals have
limited resources that must be balanced between reproduction, maintenance, and
growth. Hormones involved with reproduction change during the life span, with
high levels present prenatally, very low levels during childhood, and then major
changes occurring at puberty (Cameron 2003 ). The hormonal changes at puberty
are involved in a major, and permanent, rewiring of brain circuits (Sisk and Zehr
2005 ). Behavioral changes from the combination of neural organizational changes
and short-term pubertal hormonal variation are evident in animals, from copulatory
behavior (Söderson et al. 1977 ) to scent marking and other territorial behavior
(Ferris et al. 1987 ; Primus and Kellogg 1989 ). There is also evidence that hormonal
changes during menstrual cycles in humans are related to reproductive behavior,
with more sexual activity occurring around the time of ovulation (Brown et al.
2011 ; Harvey 1987 ; Prasad et al. 2014 ; Wilcox et al. 2004 ).
Other social behaviors in mammals are also influenced by physiology—partic-
ularly by the neuropeptides oxytocin, vasopressin, and corticotropin-releasing
factor (CRF), as well as by such hormones as testosterone, progesterone, estrogen,
and cortisol (Carter et al. 2009 ). These neuropeptides (Sanchez et al. 2009 ) and
hormones (Fleming et al. 1997 ; McIntyre and Hooven 2009 ) are biological mea-
sures of human physiological experience that help to explain human social
behavior.
However, indicators of behavior are not sufficient to fully understand human
experience. It has been long understood that the perception of an external object
may lead to different subjective experiences among individuals, with the most
subjective of these experiences labeled“affect”by William Wundt (Wundt 1897 ;
Chikazoe et al. 2014 ). An early attempt to go beyond behavior was the work by
James and Lange on the experience of emotions in humans. The James–Lange
theory states that human emotion stems from sensing the physiological changes that
come with arousal, due to stimuli that elicit muscular, hormonal, and circulatory
changes (James 1894 ; Lange 1922 ). The theory presents a peripheral locus of
emotions: the emotions derive from sensing physiological responses outside the
central nervous system. Previously, emotions had been viewed as primary elements
1 Making Visible the Invisible 3