Is "sick" the right word?

 

Ted Bundy, 1978, State Archives of Florida (Wikicommons). Outwardly charming but zero concern for others.

 

Is sociopathy an illness? We often think so ... to the point that the word "sick" has taken on a strange secondary meaning. If we call a ruthless, self-seeking person "sick," we mean he should be shunned at all costs. We don't mean he should take an aspirin and get some rest.

Sociopathy doesn't look like a mental illness, being much less incapacitating than schizophrenia and most mental disorders. A sociopath can deal with other people well enough, perhaps too well. As Harpending and Sobus (2015) point out:

It is a psychopathology because of what sociopaths do to us, and it has significant legal, political, and moral consequences for all of us. Most criminals are probably sociopaths according to some definition (the figure of 80% is often quoted).

Sociopaths regularly present the following characteristics:

- onset before age 15, childhood hyperactivity, truancy, delinquency, disruption in school

- early and often aggressive sexual activity, marital histories of desertion, non-support, abandonment
- persistent lying, cheating, irresponsibility without visible shame
- sudden changes of plan, impulsiveness, unpredictability
- charm and a façade of sensitivity
- high mobility, vagrancy, use of aliases

Sociopaths follow a life strategy that is adaptive for themselves but ruinous for society. Harpending and Sobus (2015) argue that they succeed so well because they know how to manipulate social relationships to their advantage.  

Sociopathy is at least moderately heritable (Hicks et al., 2004). Interestingly, it seems to cluster with hysteria in first-degree relatives, with sociopathy being expressed in the males and hysteria in the females. Harpending and Sobus (2015) argue that "hysteria is the expression in females of the same genetic material that leads to sociopathy in males." In short, "sociopathy in females is the result of a greater dose of the genetic material that leads, in smaller doses, to hysteria, namely, hysteria is mild sociopathy." 

If sociopathy is adaptive, why does it affect only a minority of us? It seems that the rest of us have developed counter-strategies of looking for signs of sociopathy and expelling suspects from society ... and the gene pool. This is probably why sociopaths tend to be always on the move—if they stay too long with the same people, they risk being detected and dealt with.

Gene-culture coevolution

We adapt to our cultural environment as we do to our natural environment. More so in fact. The last 10,000 years have seen far more genetic change in our ancestors than the previous 100,000, this speeding up of evolution being driven by the entry of humans into an increasingly diverse range of cultural environments.

Sociopathy may thus propagate itself more easily in some cultures than in others, with the result that its incidence may likewise differ from one to another. In a small band of hunter-gatherers, a sociopath will not last long because he is always interacting with the same small group of people:

In a 1976 study anthropologist Jane M. Murphy, then at Harvard University found that an isolated group of Yupik-speaking Inuits near the Bering Strait had a term (kunlangeta) they used to describe "a man who ... repeatedly lies and cheats and steals things and ... takes sexual advantage of many women—someone who does not pay attention to reprimands and who is always being brought to the elders for punishment." When Murphy asked an Inuit what the group would typically do with a kunlangeta, he replied, "Somebody would have pushed him off the ice when nobody else was looking." (Lilienfeld and Arkowitz, 2007)

In a larger community, a sociopath may evade detection long enough to reproduce successfully and pass on his mental traits. Finally, in some cultures he can use his manipulative skills to dominate the community, becoming a "big man" and enjoying very good opportunities for reproduction.

This Pandora's Box was opened when humans gave up hunting and gathering and became farmers. First, farming supported a much larger population, so it became easier for sociopaths to move about from one group of unsuspecting people to another. Second, farming created a food surplus that powerful individuals could use to support underlings of various sorts: servants, soldiers, scribes, etc. There was thus a growing class of people who did not directly support themselves and whose existence depended on their ability to manipulate others.

Finally, in the tropical zone, farming greatly increased female reproductive autonomy. Through year-round farming, women could provide for themselves and their children with less male assistance. Men accordingly shifted their reproductive strategy from monogamy to polygyny, i.e., from providing for a wife and children to inseminating as many women as possible. This kind of cultural environment selected for male seducers and manipulators rather than male providers. Conversely, it selected for women who feel only an intermittent need for male companionship and who from time to time are able to coax assistance from people who are not so inclined:

Ethnographic descriptions of women who live in social contexts of low male parental investment portray women who are very demanding. Young women demand help from kin on behalf of children. When the help is not forthcoming the mothers often summarily dump or deposit the child or children at the door of a relative who (in their judgment) will not turn the children away. Women demand gifts from boyfriends for themselves. (Harpending and Draper, 1988)

In women, this selection pressure favors a condition known medically as Briquet's syndrome and more commonly as "hysteria":

When males are not good risks for parental investment, females will adjust their behavior accordingly. A common clinical characterization of Briquet's syndrome is a woman who exaggerates need, who demands high levels of attention and investment, who deceives herself and others as to her requirements. The strategy (learned or inherited) makes sense for a woman with high exposure to low investment males. These males, however, are so fickle and so mobile that they can be dunned only in the short run. (Harpending and Draper, 1988)

Sociopathic behavior, be it hysteria or full-blown sociopathy, is not favored in hunter-gatherers, since both sexes invest heavily in their offspring and in each other. The selection is for men and women who can bond strongly with one partner:

[...] abandonment of the pair bond by either partner is likely to be deadly for the offspring. Draper (manuscript) finds that men with more children spend more time hunting than men with fewer dependents; that is to say that more offspring are directly translated into more parental work for the male. Pennington and Harpending (manuscript) found that infant mortality among women who had more than one mate during their reproductive careers was nearly twice as great as infant mortality of women who had only one husband. [...] In societies of this type the contexts for the anti-social trait are unfavorable. There will be no pay-offs for anti-social behavior and the bearer of the trait will be readily detected and ostracized. (Harpending and Draper, 1988)

Strategy and counter-strategy

Sociopathy is therefore not an illness but a strategy. It has been least successful in small societies where both sexes invest heavily in care for their partners and offspring. It has been more successful in larger societies, particularly those where men invest less in partners and offspring. Indeed, because sociopathy does so well in such contexts, it may have hindered the development of larger and more complex societies.

In most large societies, people seek out and expel sociopaths from their local kin group and treat everyone else with suspicion. The result is the "amoral familialism" we see throughout much of the world. People prefer to deal with relatives, hire only relatives for their businesses and, as a rule, act morally only towards relatives. Thus, the high-trust environment of the family cannot extend to society in general. Among other things, this is why the market economy has failed to develop spontaneously over most of the world and over most of history. Without strong-armed government intervention (military pacification, police, courts, etc.), markets remain marketplaces—places of exchange that are highly localized in space and time. The market principle cannot spread to most economic transactions.

Some humans have resolved this problem by freeing themselves from the straitjacket of kinship, by adhering to social rules that apply to everyone, and by ruthlessly expelling rule breakers wherever they may be. This is the adaptation that Europeans have developed to the north and west of the Hajnal line. The relative weakness of kinship ties and, correspondingly, the relative strength of individualism favored a complex of psychological traits that may be summarized as follows:

- capacity to internalize punishment for disobedience of social rules (guilt proneness).

- capacity to simulate and then transfer to oneself the emotional states of other people, especially when such people are affected by rule-breaking either by oneself or by others (affective empathy).

- tendency to frame moral rules in universal, absolute terms, i.e., moral universalism and moral absolutism, as opposed to situational morality based on kinship. Rule-breakers are likewise condemned in absolute terms and may be expelled from the entire community, as opposed to being ostracized by close kin.

The above mental package brought Northwest Europeans closer than other humans to the threshold where one could escape the limitations of kinship and organize society along other lines, notably the market economy, the modern State, and political ideology. It thus became possible to meet the challenge of creating larger societies while ensuring compliance with social rules and a high degree of personal autonomy.

References 

Cooke, D.J. (2003). Cross-Cultural Aspects of Psychopathy, in T. Millon, E. Simonsen, M. Birket-Smith, and R.D. Davis (eds). Psychopathy: Antisocial, Criminal, and Violent Behavior, pp. 260-276, Guilford Press.

https://books.google.ca/books?hl=fr&lr=&id=LSiBsdxcGigC&oi=fnd&pg=PA260&dq=psychopathy+cross-cultural&ots=nnV3xh4mZZ&sig=FBW4rme_w0tj2REoHQJpqXXSB6Q#v=onepage&q=psychopathy%20cross-cultural&f=false 

Harpending, H. and P. Draper. (1988). Antisocial behavior and the other side of cultural evolution, in T. E. Moffitt and S.A. Mednick (eds). Biological contributions to crime causation, pp. 110-125, Boston: Nijhoff.

https://books.google.ca/books?hl=fr&lr=&id=Vw7tCAAAQBAJ&oi=fnd&pg=PA293&ots=44dHOZn08j&sig=-2vXa0210OA82u-9Y-HzeEWM-y4#v=onepage&q&f=false 

Harpending, H. and J. Sobus. (2015). Sociopathy as an adaptation, Ethology and Sociobiology, 8, 63-72

https://www.academia.edu/11700522/Sociopathy_as_an_adaptation 

Hicks, B.M., R.F. Krueger, W.G. Iacono, M. McGue, C.J. Patrick. (2004). Family transmission and heritability of externalizing disorders. A twin-family study, Archives of General Psychiatry, 61, 922-928.

http://archpsyc.jamanetwork.com/article.aspx?articleid=482057 

Lilienfeld, S.O. and H. Arkowitz. (2007). What "Psychopath" Means, Scientific American, 18, 90-91.

http://faculty.fortlewis.edu/burke_b/Abnormal/Abnormal%20Readings/Psychopathy.pdf

Are liberals and conservatives differently wired?

 

Anti-UKIP protest in Edinburgh (source: Brian McNeil, Wikicommons). "Conservative" increasingly means pro-white.

 

Are liberals and conservatives differently wired? It would seem so. When brain MRIs were done on 90 young adults from University College London, it was found that self-described liberals tended to have more grey matter in the anterior cingulate cortex, whereas self-described conservatives tended to have a larger right amygdala. These results were replicated in a second sample of young adults (Kanai et al., 2011).

The amygdala is used to recognize fearful facial expressions, whereas the anterior cingulate cortex serves to monitor uncertainty and conflict (Adolphs et al., 1995; Botvinick et al., 1999; Critchley et al., 2001; Kennerley et al., 2006). Perhaps unsurprisingly, these findings were changed somewhat in the popular press. "Conservatives Big on Fear, Brain Study Finds," ran a headline in Psychology Today. The same article assured its readers that the anterior cingulate cortex "helps people cope with complexity" (Barber, 2011).

A study on 82 young American adults came to a similar conclusion. Republicans showed more activity in the right amygdala, and Democrats more activity in the left insula. Unlike the English study, the anterior cingulate cortex didn't differ between the two groups (Schreiber et al., 2013).

It would seem, then, that conservatives and liberals are neurologically different. Perhaps certain political beliefs will alter your mental makeup. Or perhaps your mental makeup will lead you to certain political beliefs. But how can that be when conservatism and liberalism have changed so much in recent times, not only ideologically but also electorate-wise? A century ago, English "conservatives" came from the upper class, the middle class, and outlying rural areas. Today, Britain's leading "conservative" party, the UKIP, is drawing more and more of its members from the urban working class—the sort of folks who routinely voted Labour not so long ago. Similar changes have taken place in the U.S. Until the 1950s, white southerners were overwhelmingly Democrats. Now, they're overwhelmingly Republicans.

Of course, the above studies are only a few years old. When we use terms like "conservative" and "liberal" we refer to what they mean today. Increasingly, both terms have an implicitly ethnic meaning. The UKIP is becoming the native British party, in opposition to a growing Afro-Asian population that votes en bloc for Labour. Meanwhile, the Republicans are becoming the party of White Americans, particularly old-stock ones, in opposition to a Democrat coalition of African, Hispanic, and Asian Americans, plus a dwindling core of ethnic whites.

So are these brain differences really ethnic differences? Neither study touches the question. The English study assures us that the participants were homogeneous:

We deliberately used a homogenous sample of the UCL student population to minimize differences in social and educational environment. The UK Higher Education Statistics Agency reports that 21.1% of UCL students come from a working-class background. This rate is relatively low compared to the national average of 34.8%. This suggests that the UCL students from which we recruited our participants disproportionately have a middle-class to upper-class background. (Kanai et al., 2011)

Yes, the students were largely middle-class, but how did they break down ethnically? Wikipedia provides a partial answer:

In 2013/14, 12,330 UCL students were from outside the UK (43% of the total number of students in that year), of whom 5,504 were from Asia, 3,679 from the European Union ex. the United Kingdom, 1,195 from North America, 516 from the Middle East, 398 from Africa, 254 from Central and South America, and 166 from Australasia (University College London, 2014)

These figures were for citizenship only. We should remember that many of the UK students would have been of non-European origin. 

We know more about the participants in the American study. They came from the University of California, San Diego, whose student body at the time was 44% Asian, 26% Caucasian, 10% Mexican American, 10% unknown, 4% Filipino, 3% Latino/Other Spanish, and 2% African American (Anon, 2010). This ethnic breakdown mirrors the party breakdown of the participants: 60 Democrats (72.5%) and 22 Republicans (27.5%).

Affective empathy and ethnicity

In my last post, I cited a study showing that the amygdala is larger in extraordinary altruists—people who have donated one of their kidneys to a stranger. In that study, we were told that a larger amygdala is associated with greater responsiveness to fearful facial expressions, i.e., a greater willingness to help people in distress. Conversely, psychopaths have a smaller amygdala and are less responsive to fearful faces (Marsh et al., 2014).

Hmm ... That's a tad different from the spin in Psychology Today. Are liberals the ones who don't care about others? Are they ... psychopaths?

It would be more accurate to say that "liberals" come from populations whose capacity for affective empathy is lower on average and who tend to view any stranger as a potential enemy. That's most people in this world, and that's how most of the world works. I suspect the greater ability to monitor uncertainty and conflict reflects adaptation to an environment that has long been socially fragmented into clans, castes, religions, etc. This may explain why a larger anterior cingulate cortex correlated with "liberalism" in the British study (high proportion of South Asian students) but not in the American study (high proportion of East Asian students).

As for "conservatives," they largely come from Northwest Europe, where a greater capacity for affective empathy seems to reflect an environment of relatively high individualism, relatively weak kinship, and relatively frequent interactions with nonkin. This environment has prevailed west of the Hajnal Line since at least the 12th century, as shown by the longstanding characteristics of the Western European Marriage Pattern: late age of marriage for both sexes; high rate of celibacy; strong tendency of children to form new households; and high circulation of non-kin among families. This zone of weaker kinship, with greater reliance on internal means of behavior control, may also explain why Northwest Europeans are more predisposed to guilt than to shame, whereas the reverse is generally the case elsewhere in the world (Frost, 2014).

All of this may sound counterintuitive. Doesn't the political left currently stand for autonomy theory and individualism? Doesn't it reject traditional values like kinship? In theory it does. The reality is a bit different, though. When Muslims vote Labour, it's not because they want gay marriage and teaching of gender theory in the schools. They expect something else.

The same goes for the political right. When former Labourites vote UKIP, it's not because they want lower taxes for the rich and offshoring of manufacturing jobs. They expect something else. Are they being delusional? Perhaps. But, then, are the Muslims being delusional? 

Perhaps neither group is. Perhaps both understand what politics is really about.

 

References

 

Adolphs, R., D. Tranel, H. Damasio, and A.R. Damasio. (1995). Fear and the human amygdala, The Journal of Neuroscience, 15, 5879-5891.

http://www.emotion.caltech.edu/papers/AdolphsTranel1995Fear.pdf 

Anon (2010). Racial breakdown of the largest California public colleges, The Huffington Post, May 4

http://www.huffingtonpost.com/2010/03/04/racial-breakdown-of-the-l_n_485577.html 

Barber, N. (2011). Conservatives big on fear, study finds, Psychology Today, April 19

http://www.psychologytoday.com/blog/the-human-beast/201104/conservatives-big-fear-brain-study-finds

Botvinick, M., Nystrom, L.E., Fissell, K., Carter, C.S., and Cohen, J.D. (1999). Conflict monitoring versus selection-for-action in anterior cingulate cortex, Nature, 402, 179-181.

Critchley, H.D., Mathias, C.J., and Dolan, R.J. (2001). Neural activity in the human brain relating to uncertainty and arousal during anticipation, Neuron, 29, 537-545. 

Frost, P. (2014). We are not equally empathic, Evo and Proud, November 15

http://www.evoandproud.blogspot.ca/2014/11/we-are-not-equally-empathic.html 

Kanai, R., T. Feilden, C. Firth, and G. Rees. (2011). Political orientations are correlated with brain structure in young adults, Current Biology, 21, 677 - 680.

http://www.cell.com/current-biology/abstract/S0960-9822(11)00289-2

Kennerley, S.W., Walton, M.E., Behrens, T.E., Buckley, M.J., and Rushworth, M.F. (2006). Optimal decision making and the anterior cingulate cortex. Nat. Neurosci. 9, 940-947.

Marsh, A.A., S.A. Stoycos, K.M. Brethel-Haurwitz, P. Robinson, J.W. VanMeter, and E.M. Cardinale. (2014). Neural and cognitive characteristics of extraordinary altruists, Proceedings of the National Academy of Sciences, 111, 15036-15041.

http://www.pnas.org/content/111/42/15036.short

Schreiber, D., Fonzo, G., Simmons, A.N., Dawes, C.T., Flagan, T., et al. (2013). Red Brain, Blue Brain: Evaluative Processes Differ in Democrats and Republicans. PLoS ONE 8(2): e52970.

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0052970 

University College London. (2014). Wikipedia

http://en.wikipedia.org/wiki/University_College_London#Student_body

We are not equally empathic

 

The Child at Your Door (c. 1917-1919). We're not equally empathic toward strangers. This largely heritable trait varies continuously from psychopathy to extraordinary altruism (source: Wikicommons)

 

In a previous post, I discussed why the capacity for affective empathy varies not only between individuals but also between populations. First, its heritability is high: 68% (Chakrabarti and Baron-Cohen, 2013). So natural selection has had something to grab hold of. Second, its usefulness varies from one culture to another. It matters less where kinship matters more, i.e., where people interact mainly with close kin and where non-kin are likely to be enemies. The threat of retaliation from kin is sufficient to ensure correct behavior.

Affective empathy matters more where kinship matters less. This is a situation that Northwest Europeans have long known. Historian Alan Macfarlane argues that kinship has been weaker among the English—and individualism correspondingly stronger—since at least the 12th century and perhaps since Anglo-Saxon times (Macfarlane, 2012; Macfarlane, 1992, pp. 173-174). A weaker sense of kinship seems to underlie the Western European Marriage Pattern (WEMP), as seen by its defining characteristics: late age of marriage for both sexes; high rate of celibacy; strong tendency of children to form new households; and high circulation of non-kin among families. The WEMP has prevailed since at least the 12th century west of the Hajnal Line, a line running approximately from Trieste to St. Petersburg (Hallam, 1985; Seccombe, 1992, p. 94).

Can natural selection specifically target affective empathy?

So if affective empathy helps people to survive and reproduce, there will be more and more of it in succeeding generations. If not, there will be less and less.

But what exactly is being passed on or not passed on? A specific capacity? Or something more general, like pro-social behavior? If it's too general, natural selection could not easily make some populations more altruistic than others. There would be too many nasty side-effects.

Although pro-social behavior superficially looks like affective empathy, the underlying mental processes are different. Pro-social behavior is a willingness to help others through low-cost assistance: advice, conversation, a helping hand, etc. The logic is simple: give some help now and perhaps you'll receive a lot later from the grateful beneficiary. By the same logic, you may stop helping someone who seldom reciprocates.

Affective empathy is less conscious. It seems to have developed out of cognitive empathy: the ability to simulate what is going on in other people's minds, but not necessarily for the purpose of helping them. Con artists have plenty of cognitive empathy. Empathy is affective when you not only simulate how other people feel but also experience their feelings (Chakrabarti and Baron-Cohen,2013). Their wellbeing comes to matter as much as your own. 

Empathy of either sort relies on unconscious mimicry: "empathic individuals exhibit nonconscious mimicry of the postures, mannerisms, and facial expressions of others (the chameleon effect) to a greater extent than nonempathic individuals" (Carr et al., 2003). The ability to mimic is key to the empathic process of relaying information from one brain area to another via "mirror neurons":

- The superior temporal cortex codes an early visual description of another person's action and sends this information to posterior parietal mirror neurons.

- The posterior parietal cortex codes the precise kinesthetic aspect of the action and sends the information to inferior frontal mirror neurons.

- The inferior frontal cortex codes the purpose of the action.

- Parietal and frontal mirror areas send copies of motor plans back to the superior temporal cortex in order to match the visual description of the person's action to the predicted sensory consequences for that person.

- The mental simulation is complete when the visual description has been matched to the predicted sensory consequences (Carr et al., 2003).

By simulating the sensory consequences of what someone does or intends to do, we gain an understanding of that person that goes beyond what our senses immediately tell us. 

[...] we understand the feelings of others via a mechanism of action representation shaping emotional content, such that we ground our empathic resonance in the experience of our acting body and the emotions associated with specific movements. As Lipps noted, ''When I observe a circus performer on a hanging wire, I feel I am inside him.'' To empathize, we need to invoke the representation of the actions associated with the emotions we are witnessing. (Carr et al., 2003)

Affective empathy exists when this mental representation is fed into our own emotional state. We feel what the other person feels and we act appropriately. This is much more than pro-social behavior.

From psychopaths to extraordinary altruists

The capacity for affective empathy varies from one person to the next. It is least developed in psychopaths:

Psychopathy is a heritable developmental disorder characterized by an uncaring nature, antisocial and aggressive behavior, and deficient prosocial emotions such as empathy, guilt, and remorse. Psychopaths exhibit consistent patterns of neuroanatomical and functional impairments, such as reductions in the volume of the amygdala and in the responsiveness of this structure to fear-relevant stimuli. These deficits may underlie the perceptual insensitivity to fearful facial expressions and other fear-relevant stimuli observed in this population. (Marsh et al., 2014)

Mainstream opinion accepts that psychopaths are heritably different because they are "sick." Heritable differences are thus thought to be unusual and even pathological. "Normal" individuals may vary in their capacity for affective empathy, but surely that sort of variability is due to their environment, isn't it?

No it isn't. That variability, too, is largely genetic. Affective empathy varies over a largely heritable continuum, and an arbitrary line is all that separates psychopaths from "normal" individuals. There may be many psychopaths or there may be few; it depends on where you set the cut-off point.

At the other end of this continuum is another interesting group: extraordinary altruists. A research team has recently looked at the brains of such people, specifically individuals who had donated one of their kidneys to a stranger:

Given emerging consensus that psychopathy is a continuously distributed variable within the general population and that psychopaths represent one extreme end of a caring continuum, we hypothesized that extraordinary altruism may represent the opposite end of this continuum and be supported by neural and cognitive mechanisms that represent the inverse of psychopathy; in particular, increased amygdala volume and responsiveness to fearful facial expressions. (Marsh etal., 2014)

In extraordinary altruists, the right amygdala is larger and responds more to fearful facial expressions. This is the inverse of what we see in psychopaths, who have smaller amygdala and are less responsive to fearful facial expressions.

Affective empathy is thus a specific mental trait, like psychopathy. It is not a form of pro-social behavior any more than psychopathy is a form of antisociality:

[...] it is important to distinguish between antisociality that results from psychopathy, which is specifically associated with reduced empathy and concern for others, as well as with reduced sensitivity to others' fear and distress, and antisociality that results from any of a variety of other factors, such as impulsivity or trauma exposure, that are not closely related to empathy. (Marshet al., 2014)

Marsh et al. (2014) cite a number of studies to show the relative independence of these two behavioral axes: prosociality / antisociality and affective empathy / psychopathy.

Conclusion

Affective empathy is specific and largely heritable. People differ continuously in their innate capacity for affective empathy, and it is only by setting an arbitrary cut-off point that we classify some as "psychopaths" and others as "normal," including extraordinary altruists who may be a small minority.

Affective empathy is an intricate adaptation that must have evolved for some reason. Initially, it may have served to facilitate the relationship between a mother and her children, this being perhaps why it is stronger in women than in men (Baron-Cohen and Wheelwright, 2004). In some cultures, natural selection may have increased this capacity in both sexes and extended it to a wider range of social interactions. This scenario would especially apply to Northwest Europeans, who have long had relatively weak kinship. They have consequently relied more on internal means of behavior control, like affective empathy (Frost, 2014).

 

References 

Baron-Cohen, S. and S. Wheelwright. (2004).The Empathy Quotient: An investigation of adults with Asperger Syndrome or high functioning autism, and normal sex differences. Journal of Autism and Developmental Disorders, 34, 163-175.

http://ftp.aspires-relationships.com/the_empathy_quotion_of_adults_with_as.pdf 

Carr, L., M. Iacoboni, M-C. Dubeau, J.C. Mazziotta, and G.L. Lenzi. (2003). Neural mechanisms of empathy in humans: A relay from neural systems for imitation to limbic areas, Proceedings of the National Academy of Sciences (USA), 100, 5497-5502.

http://www.ucp.pt/site/resources/documents/ICS/GNC/ArtigosGNC/AlexandreCastroCaldas/7_CaIaDuMaLe03.pdf  

Chakrabarti, B. and S. Baron-Cohen. (2013). Understanding the genetics of empathy and the autistic spectrum, in S. Baron-Cohen, H. Tager-Flusberg, M. Lombardo. (eds). Understanding Other Minds: Perspectives from Developmental Social Neuroscience, Oxford: Oxford University Press.

http://books.google.ca/books?hl=fr&lr=&id=eTdLAAAAQBAJ&oi=fnd&pg=PA326&ots=fHpygaxaMQ&sig=_sJsVgdoe0hc-fFbzaW3GMEslZU#v=onepage&q&f=false 

Frost, P. (2014). Affective empathy. An evolutionary mistake?  Evo and Proud, September 20

http://evoandproud.blogspot.ca/2014/09/affective-empathy-evolutionary-mistake.html

Hallam, H.E. (1985). Age at first marriage and age at death in the Lincolnshire Fenland, 1252-1478, Population Studies, 39, 55-69. 

Macfarlane, A. (1992). On individualism, Proceedings of the British Academy, 82, 171-199.

http://www.alanmacfarlane.com/TEXTS/On_Individualism.pdf 

Macfarlane, A. (2012). The invention of the modern world. Chapter 8: Family, friendship and population, The Fortnightly Review, Spring-Summer serial

http://fortnightlyreview.co.uk/2012/07/invention-8/ 

Marsh, A.A., S.A. Stoycos, K.M. Brethel-Haurwitz, P. Robinson, J.W. VanMeter, and E.M. Cardinale. (2014). Neural and cognitive characteristics of extraordinary altruists, Proceedings of the National Academy of Sciences, 111, 15036-15041.

http://www.pnas.org/content/111/42/15036.short

Seccombe, W. (1992). A Millennium of Family Change. Feudalism to Capitalism in Northwestern Europe, London: Verso.