Abstract: Our long co-evolutionary history with infectious agents likely began soon after the rise of the first single-celled organisms. This ongoing evolutionary arms race has generated complex host adaptations, many highly conserved, for resisting infection (e.g., innate and acquired immune systems, infection-sensitive developmental programs, sexual reproduction). A large body of evidence suggests that, in humans, pathogen-avoidance disgust is an emotion that motivates avoidance of cues associated with pathogens, thereby reducing infection. However, the question of whether there is an emotion that coordinates resistance to active infection has received less attention. We propose that lassitude is such an emotion. It is triggered by cues of active infection and coordinates the fight against infection by: (a) reducing energetically expensive movement to make more energy available to the immune system, (b) reducing exposure to additional infections and injuries that would compound the immune system's workload, (c) promoting thermoregulatory behaviors that facilitate immunity, (d) regulating food consumption to be beneficial for the host but detrimental to pathogens, and (e) deploying strategies that elicit caregiving behavior from social allies. Lassitude exhibits the core features of an emotion – it is triggered by cues of an adaptive problem (i.e., infection), generates a characteristic facial expression (e.g., slack facial muscles, drooping eyelids, slightly parted lips), and has distinct qualia (e.g., profound tiredness, reduced appetite, feelings of vulnerability, altered temperature perception, increased pain sensitivity). We outline the information-processing structure of lassitude, review existing evidence, suggest directions for future research, and discuss implications of lassitude for models of human evolution.
5. Lassitude: a new emotion?
Our
approach to characterizing lassitude is informed by Tooby and Cosmides'
framework: “[the emotions] are the neurocomputational adaptations that
have evolved in response to the adaptive problem of matching arrays of
mechanism activation to the specific adaptive demands imposed by
alternative situations” (Tooby & Cosmides, 2008,
p. 117). Lassitude satisfies this definition of an emotion. It is a
coordinating system that functions to orchestrate various mechanisms to
solve the adaptive problem of fighting infectious disease.
Other theoretical approaches emphasize the distinctive facial expression and qualia of an emotion (Ekman & Oster, 1979; Frijda, 2005).
We propose that lassitude has a distinctive facial expression generated
by less muscle tension relative to a neutral facial expression (i.e.,
slack facial muscles). In particular, it consists of a long
crown-to-chin length, drooping eyelids, and slightly parted lips. A
recent study showed participants pictures of faces of people who had
been injected with LPS or with placebo (Axelsson et al., 2018).
Participants correctly identified the faces of sick people at a higher
rate than chance, even though individuals in the photos were instructed
to exhibit a neutral facial expression, which may have resulted in
partial masking of the facial expression of lassitude. The faces of sick
people were rated as having droopy corners of the mouth, having hanging
eyelids, looking more tired, having redder eyes, and having paler,
puffier skin. We propose that lassitude also has distinct qualia –
profound tiredness, greater relative preference for close social allies,
reduced overall appetite but stronger relative preference for
particular food items, greater feelings of vulnerability, increased pain
sensitivity, greater susceptibility to nausea, and altered perceptions
of ambient temperature. Not all of these qualia are present in all cases
of infection – the presence of each component depends on
context-sensitive regulation of the underlying motivational system.
Panksepp's account of basic emotions emphasizes universality across species of mammals and conserved neural circuitry (Panksepp, 1982).
Lassitude involves specific immune and neural signaling pathways that
have been mapped with increasing thoroughness in animal models (McCusker & Kelley, 2013). Lassitude-like regulatory states exist not only in mammals but also birds (Owen-Ashley & Wingfield, 2007), amphibians (Llewellyn et al., 2011), and possibly even fish (Kirsten, Fior, Kreutz, & Barcellos, 2018).
Based
on the criteria discussed here, lassitude is an emotion, albeit one
that has gone unrecognized. It is worth noting that there are at least
two ways in which our account of lassitude is not novel. First, Tooby
and Cosmides hypothesized that malaise during infection might be an
evolved emotion but did not elaborate (Tooby & Cosmides, 2008).
Second, scholars who study sickness behavior, focusing mostly on
non-humans, have proposed that many infection-induced behavioral changes
are evolutionarily adaptive (Adelman & Martin, 2009; Hart, 1990).
However, the sickness behavior literature largely focuses on the
description of infection-related behavior and its physiological
mechanisms. It has largely neglected to characterize the evolutionary
background, information-processing structure, and functional logic of
the regulatory system that generates sickness behavior.
6. Directions for future research
6.1. Social support and immunity
Humans
provide care to social allies during illness and injury (see section
2.2.5). This is an important buffer against the opportunity costs of
reducing movement when sick. A compelling hypothesis to explain the
placebo effect is that one function of visible illness symptoms is to
elicit care from others (Steinkopf, 2015).
When others provide care, the signaling function of the symptoms is
fulfilled and the symptoms become less severe (ibid). We predict that
cues of social support (or a lack thereof) are also key inputs for
modulating the regulation of lassitude during infection. Infected
individuals who are socially isolated may be unable to afford to devote
as much energy to fighting infection, and may therefore experience
longer-lasting infections and higher mortality risks. In support of this
hypothesis, there is evidence to suggest that those who are socially
isolated tend to suffer from poorer health (Cacioppo & Cacioppo, 2014).
6.2. Lassitude in healthcare settings
We
hypothesize that cues of infection increase the relative preference for
contact with social allies. Furthermore, we propose that sick
individuals boost signals of vulnerability when in the presence of
social allies (in order to elicit care) and mask signals of
vulnerability in front of strangers and antagonists (in order to reduce
the risk of social and physical danger during the vulnerable state of
sickness). This suggests that the degree to which a patient sees a
healthcare provider as a social ally may have a major influence on a
patient's decision to pursue care, and ability to elicit it.
Patients
who do not see providers as social allies may be less likely to seek
healthcare, and when they do, may have greater difficulty eliciting
useful care, due to the fact that they are (perhaps inadvertently)
masking signals of vulnerability. This may help explain why many
patients highly value good “bedside manner” in healthcare providers (Thompson & Anderson, 1982).
6.3. Lassitude and chronic disease
There
is evidence to suggest that chronic diseases (e.g., heart disease,
diabetes, chronic obstructive pulmonary disorder) may activate a
response that resembles a chronic version of lassitude (Swain, 2000).
This may be due, in part, to the fact that chronic somatic damage
activates some of the same pro-inflammatory immune pathways that trigger
lassitude during infection (Del Giudice & Gangestad, 2018; McCusker & Kelley, 2013).
This poses a problem because one of most effective interventions for
preventing and treating chronic disease is to engage in healthy levels
of physical activity (Warburton, Nicol, & Bredin, 2006).
The motivational state of lassitude directly opposes this goal. Thus,
even sub-clinical levels of chronic morbidity may trigger a vicious,
self-reinforcing cycle in which greater chronic morbidity leads to
greater lassitude, and greater lassitude leads to even greater chronic
morbidity. This cycle may help explain why chronic disease epidemics
emerge when populations transition to economic sectors with a greater
proportion of sedentary occupations (Omran, 2005).
6.4. Lassitude in relation to time, reward, and risk preferences
We
hypothesize that lassitude modifies the cost-benefit structure of a
wide range of decisions. Individuals in a state of lassitude place a
lower value on some types of rewards (e.g., food, sex). Higher levels of
lassitude may therefore generate a greater willingness to delay some
kinds of payoffs in temporal discounting scenarios (Green, Myerson, & Mcfadden, 1997).
We also propose that individuals in a state of lassitude place a
greater value on avoiding social and physical risks. Thus, lassitude may
induce greater risk aversion when the risks are social or physical. On
the other hand, lassitude may induce less aversion to the risk
of losing a potential payoff that has lower value during lassitude
(e.g., food, sex). Researchers who study time-, reward-, and
risk-related decision-making should therefore consider incorporating
lassitude into their studies and theoretical models.
6.5. Approach-avoidance conflict
The
fact that humans systematically help social allies during illness
suggests that we have cognitive mechanisms for detecting signs of
illness in others and deciding how to respond. The occurence of illness
in a social conspecific poses a potential motivational conflict. On one
hand, helping the sick individual may induce the target of aid to
reciprocate in the future, when the roles are reversed (Gurven et al., 2000; Sugiyama, 2004; Sugiyama & Chacon, 2000). Furthermore, helping sick individuals provides a costly signal of the helper's quality as a social ally (Steinkopf, 2015).
On the other hand, the effort required for providing aid is costly, and
providing aid to a sick person may increase the helper's risk of
getting sick (Tybur et al., 2012).
Thus, navigating the motivational conflict between approaching a sick
individual (to help them) and avoiding them (to reduce infection risk)
is an important adaptive problem for humans. Variables that influence
the decision to help or avoid a sick person may include the
relationship/kinship of the sick person to the helper, the sick person's
reputation as a reciprocator, the helper's vulnerability to illness,
and the social audience that would be aware of the helping behavior.
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