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Morality, a by-product of natural selection?

Pierre-Marie Lledo
Pierre-Marie Lledo
Research Director at CNRS, Head of Department at Institut Pasteur, and member of the European Academy of Sciences
Key takeaways
  • Recent findings show a link between cognitive neuroscience and the field of moral philosophy.
  • This research is beginning to reveal the identity of the brain circuits involved in moral judgements. We are discovering that systems for learning about reward, assessing risk and understanding the mental state of others are at work.
  • This human faculty of morality would have provided an incredible adaptive advantage. Indeed, our species is living proof that to associate is to survive.
  • Morality makes it possible to associate and to limit attacks on safety, health, social conflicts, and other dangerous deviant behaviour.
  • These early predispositions to prosocial behaviour reflect pre-wired capacities that were adaptive to our ancestors and that enabled our species to “become master and possessor of nature”.

At first sight, moral­i­ty and sci­ence are two dis­ci­plines so dis­tinct that it would be inap­pro­pri­ate to con­fuse their respec­tive roles. How­ev­er, recent dis­cov­er­ies in cog­ni­tive neu­ro­science show the two dis­ci­plines are more inter­linked than pre­vi­ous­ly thought, with the pos­si­ble involve­ment of neu­ro­science in the field of moral phi­los­o­phy. But can we reduce man to the shape­less, grey­ish organ between his ears? Is he the prod­uct of his brain’s activ­i­ty? For the fol­low­ers of sci­en­tif­ic ratio­nal­i­ties, there is no doubt that the psy­cho­log­i­cal, social, or moral self is car­ried out through oper­a­tions enabled by brain cir­cuits1. Accord­ing to this point of view, there is a nat­u­ral­is­tic inter­pre­ta­tion of moral­i­ty that I will try to explain here.

Science and Morality

Very ear­ly on, phi­los­o­phy aimed to give a def­i­n­i­tion of moral­i­ty, con­sid­er­ing it as being at the cen­tre of our actions, as if it were a ref­er­ence point for cor­rect con­duct. In oth­er words, moral­i­ty and hap­pi­ness are linked since, if the pur­pose of exis­tence is hap­pi­ness, the means to achieve it are defined by moral­i­ty. It is there­fore not sur­pris­ing to find that in all cul­tures, humans are deeply con­cerned with moral­i­ty, for exam­ple by cre­at­ing insti­tu­tions such as courts to ensure that social norms apply to all.

Research in cog­ni­tive neu­ro­science is begin­ning to reveal the iden­ti­ty of the brain cir­cuits involved in these moral judge­ments. We are dis­cov­er­ing that sys­tems for learn­ing about reward, assess­ing risk, and under­stand­ing the men­tal state of oth­ers are at work. How­ev­er, if they seem fun­da­men­tal to gen­er­ate a moral judge­ment, their degree of par­tic­i­pa­tion varies accord­ing to indi­vid­ual dif­fer­ences in terms of empa­thy, benev­o­lence, or greater or less­er sen­si­tiv­i­ty to injus­tice. In oth­er words, each indi­vid­ual has his or her own thresh­old of sen­si­tiv­i­ty to moral­i­ty. At the extreme end of this spec­trum, dis­tur­bances in the social deci­sion-mak­ing cir­cuit gen­er­ate immoral behav­iour such as that of psychopaths.

Sapiens, a species in search of justice

All mem­bers of human soci­eties, how­ev­er diverse they may appear, show a deep con­cern for issues of moral­i­ty, jus­tice and equi­ty2. Humans are dis­tin­guished from oth­er species by their propen­si­ty to pro­duce cul­tur­al organ­i­sa­tions to ensure that social norms are respect­ed, includ­ing insti­tu­tions designed to assess the accept­abil­i­ty of indi­vid­ual behav­iour and to assign appro­pri­ate pun­ish­ments to those who vio­late par­tic­u­lar norms3. Regard­less of how moral­i­ty may be con­cep­tu­alised, and fol­low­ing Darwin’s work4, researchers argue that moral judge­ment is a human char­ac­ter­is­tic that facil­i­tates coop­er­a­tion among large groups of unre­lat­ed indi­vid­u­als5. In short, humans ben­e­fit from a spe­cial rela­tion­ship with oth­ers because it is total­ly dis­in­ter­est­ed, inde­pen­dent of a fil­ial rela­tion­ship such as that which gov­erns the care of kit­tens by a cat.

The emer­gence of this men­tal fac­ul­ty would have giv­en Sapi­ens an adap­tive advan­tage that we can mea­sure today by the inva­sive nature of our species. As the mus­ke­teers used to say6, our species is liv­ing proof that liv­ing in groups improves an individual’s chances of sur­vival. Thus, moral stan­dards pro­vide safe­guards against safe­ty or health haz­ards, and the rein­force­ment of moral behav­iour min­imis­es crim­i­nal behav­iour and social con­flict. In this way, moral­i­ty makes human soci­ety a viable enterprise.

Being more moral means living longer

The exam­ple of our longevi­ty, which is close­ly linked to the qual­i­ty of social inter­ac­tion, demon­strates this. To define the nature of the main caus­es of age­ing, sci­en­tists analysed blood sam­ples tak­en from peo­ple who were more or less social. They found that extreme­ly social peo­ple had longer pro­tec­tive telom­eres. When they looked at the pres­ence of par­tic­u­lar cells, known as senes­cent cells7, social­ly iso­lat­ed peo­ple were the ones who accu­mu­lat­ed the high­est num­ber of these cells respon­si­ble for unde­sir­able events such as inflam­ma­tion. By show­ing how the rela­tion­ship with oth­ers remains the most impor­tant fac­tor in effec­tive­ly com­bat­ing the planned obso­les­cence of an indi­vid­ual, the biol­o­gy of age­ing con­firms the impor­tance of social rela­tion­ships for the sur­vival of the species. In oth­er words, by adjust­ing a sub­jec­t’s longevi­ty to the yard­stick of his or her vir­tu­ous social rela­tion­ships, our moral judg­ments and behav­iour are the guar­an­tors of social cohesion.

An evolutionary legacy

On an evo­lu­tion­ary lev­el, moral­i­ty appears to be an extreme form of coop­er­a­tion that requires indi­vid­u­als to sup­press their own inter­est or to assim­i­late it with that of oth­ers. The study of pri­mate or child behav­iour shows the con­served nature of this trait. It indi­cates that coop­er­a­tion first aris­es from a per­son­al moral­i­ty cen­tred on the indi­vid­ual and those close to him. Then, as a kind of exten­sion of this per­son­al moral­i­ty, a uni­ver­sal moral­i­ty emerges. It is from this sec­ond stage that indi­vid­u­als fol­low and apply the social norms of the group. Moral­i­ty thus emerged over the course of evo­lu­tion, becom­ing more com­plex, from a set of skills and moti­va­tions to coop­er­ate with one’s rel­a­tives at first, and then with everyone.

While ani­mals obvi­ous­ly do not explic­it­ly rea­son about right and wrong, vice and virtue, or just and unjust, some exhib­it behav­iours that seem to incor­po­rate sketch­es of vir­tu­ous behav­iour. Many species coop­er­ate, help their rel­a­tives and care for their off­spring, and some already show an aver­sion to inequal­i­ty. Sim­i­lar­ly, while social­i­sa­tion influ­ences moral devel­op­ment and explains why moral rules may evolve over space and time, human infants enter the world already equipped with cog­ni­tion and moti­va­tion that incline them to be moral and proso­cial8. These ear­ly pre­dis­po­si­tions to proso­cial behav­iour and socio-moral eval­u­a­tion reflect pre-wired capac­i­ties that were adap­tive for our ances­tors. This pen­chant for coop­er­a­tion with unre­lat­ed indi­vid­u­als would explain how our species became mas­ters and pos­ses­sors of nature9.

In sum­ma­ry, the moral con­science observed in humans appears to be a con­se­quence of sev­er­al cog­ni­tive, exec­u­tive and moti­va­tion­al capac­i­ties that are the attrib­ut­es nat­ur­al selec­tion has direct­ly favoured10. Decades of research across mul­ti­ple dis­ci­plines, includ­ing behav­iour­al eco­nom­ics, devel­op­men­tal psy­chol­o­gy and social neu­ro­science, indi­cate that moral rea­son­ing aris­es from com­plex social deci­sion-mak­ing and involves both uncon­scious and delib­er­ate process­es that rely on sev­er­al par­tial­ly dis­tinct dimen­sions, includ­ing under­stand­ing of inten­tion, aver­sion to harm, reward and val­ue cod­ing, exec­u­tive func­tion­ing and rule learn­ing11. To sum up, human moral deci­sions are gov­erned both by sta­tis­ti­cal expec­ta­tions (based on observed fre­quen­cies), Bayesian infer­ences about what oth­ers will do, and by nor­ma­tive beliefs about what oth­ers should do.

1Le grand para­doxe pour Sapi­ens est de pos­séder un cerveau dont la fonc­tion essen­tielle est de garan­tir son indi­vid­u­a­tion tout en favorisant les inter­ac­tions sociales pour for­mer un col­lec­tif homogène.
2Dece­ty J et Yoder KJ (2017). The emerg­ing social neu­ro­science of jus­tice moti­va­tion. Trends in Cog­ni­tive Sci­ences, 21(1), 6–14.
3Buck­holtz JW et Marois R (2012). The roots of mod­ern jus­tice: cog­ni­tive and neur­al foun­da­tions of social norms and their enforce­ment. Nature Neu­ro­science, 15(5), 655–661.
4Dar­win C (1871). The Descent of Man and Selec­tion in Rela­tion to Sex (Vol. 1). Lon­don, UK.
5Tomasel­lo M et Vaish A (2013). Ori­gins of Human Coop­er­a­tion and Moral­i­ty. Annu­al Review of Psy­chol­o­gy, 64, 231–255.
6Un pour tous, tous pour un, d’o­rig­ine latine et actuelle devise de la Suisse, cette expres­sion a été pop­u­lar­isée par les Trois Mous­que­taires, d’Alexan­dre Dumas.
7La sénes­cence est le proces­sus de vieil­lisse­ment biologique qui se traduit par un arrêt irréversible du cycle cel­lu­laire aboutis­sant à la mort de la cel­lule. Au bout d’un cer­tain nom­bre de divi­sions, les cel­lules finis­sent par ne plus se repro­duire et meurent.
8Ham­lin JK (2015). The infan­tile ori­gins of our moral brains In Dece­ty J & Wheat­ley T (Eds.), The Moral Brain: A Mul­ti­dis­ci­pli­nary Per­spec­tive (pp. 105–122). Cam­bridge, MA: MIT Press.
9Cf. Leçon inau­gu­rale Homo sapi­ens, une espèce inva­sive Jean-Jacques Hublin du 13 jan­vi­er 2022.
10Yoder KJ et Dece­ty J (2018). The Neu­ro­science of moral­i­ty and social deci­sion-mak­ing, Psy­chol Crime Law. 24(3), 279–295.
11Dece­ty J et Cow­ell JM (2017). Inter­per­son­al harm aver­sion as a nec­es­sary foun­da­tion for moral­i­ty: A devel­op­men­tal neu­ro­science per­spec­tive. Devel­op­ment and Psy­chopathol­o­gy, 1–12; Krueger F et Hoff­man M (2016). The emerg­ing neu­ro­science of third-par­ty pun­ish­ment. Trends in Neu­ro­sciences, 39(8), 499–501.


Pierre-Marie Lledo

Pierre-Marie Lledo

Research Director at CNRS, Head of Department at Institut Pasteur, and member of the European Academy of Sciences

Pierre-Marie Lledo’s research focuses on the adaptation and regeneration of neurons in the brain, and their interactions with the immune system. He is Research director at the CNRS, head of the Genes and Cognition laboratory, and director of the Perception and Memory unit and of Plasticity and Development of the Nervous System at the Pasteur Institute.

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