Thursday, March 25, 2021

People with schizophrenia, and possibly forensic personality disorders populations, demonstrate a range of reading skills deficits

Reading skills deficits in people with mental illness: A systematic review and meta-analysis. Martina Vanova et al. European Psychiatry, Volume 64 Issue 1, November 3 2020.


Background: Good reading skills are important for appropriate functioning in everyday life, scholastic performance, and acquiring a higher socioeconomic status. We conducted the first systematic review and meta-analysis to quantify possible deficits in specific reading skills in people with a variety of mental illnesses, including personality disorders (PDs).

Methods: We performed a systematic search of multiple databases from inception until February 2020 and conducted random-effects meta-analyses.

Results: The search yielded 34 studies with standardized assessments of reading skills in people with one or more mental illnesses. Of these, 19 studies provided data for the meta-analysis. Most studies (k = 27; meta-analysis, k = 17) were in people with schizophrenia and revealed large deficits in phonological processing (Hedge’s g = −0.88, p < 0.00001), comprehension (Hedge’s g = −0.96, p < 0.00001) and reading rate (Hedge’s g = −1.22, p = 0.002), relative to healthy controls; the single-word reading was less affected (Hedge’s g = −0.70, p < 0.00001). A few studies in affective disorders and nonforensic PDs suggested weaker deficits (for all, Hedge’s g < −0.60). In forensic populations with PDs, there was evidence of marked phonological processing (Hedge’s g = −0.85, p < 0.0001) and comprehension deficits (Hedge’s g = −0.95, p = 0.0003).

Conclusions: <. Future studies are needed to establish how these deficits directly compare to those seen in developmental or acquired dyslexia and to explore the potential of dyslexia interventions to improve reading skills in these populations.


This systematic review and meta-analysis evaluated existing evidence to identify the type and degree of reading impairments in different MIs, the reading assessment tools that might most consistently detect them, and possible differences in the pattern of reading skills deficits in people with different MIs in forensic and nonforensic settings. Most of the reviewed studies (27/34) included people with SZ. There were seven studies of reading skills deficits in people with different MIs (PD or general MI) in forensic settings. Our findings are discussed below.

Effect of diagnosis in nonforensic samples

We observed significant deficits in multiple reading skills in SZ, resembling the pattern typically seen in dyslexia [6], and consistent with previous evidence for shared genetic and psychophysiological traits in SZ and dyslexia [7]. In our meta-analysis, both phonological processing and comprehension were greatly impaired. These impairments may be associated with ineffective use of contextual information [91] and contribute to poor speech in SZ, especially in close association with thought disorder [92]. Reading rate was low but the deficit in reading accuracy was lower. This indicates relatively preserved single-word reading skills, most likely because they are usually acquired before illness onset and remain intact [47]. In contrast, there was evidence for impairments in vocabulary and spelling, presumably as a result of disrupted scholastic experience. Disrupted scholastic experience during adolescence can affect complex skills such as comprehension [44,45,47], which could precipitate difficulties with processing complex written information in SZ. People with SZ showed reading skills well below their achieved education level (see Education). Reading skills deficits in SZ also do not seem to be explained by other aspects of cognition (see Cognitive Function) although more comprehensive investigations are needed to substantiate this. Our findings (Symptoms and Medication) further indicated that while symptoms and high antipsychotic doses may worsen reading skills, they do not fully explain the profile of reading skills deficits in SZ. Impairment in comprehension and vocabulary was present even before the onset of symptoms [44,45] together with deficient phonological processing, which has been related to disrupted visual processing in SZ since early age [21]. The symptoms can, however, aggravate deficits in reading skills, such as comprehension, which are acquired with experience, and also depend on the earlier acquired skills [93]. Recent data [94] suggest that some aspects of language production (e.g., slower articulation) that can affect reading skills assessments are particularly sensitive to dopamine-D2 receptor blocking antipsychotics. Furthermore, most studies in SZ included more men than women or men solely and also included people with schizoaffective disorder. Further studies need to comprehensively examine specific reading skills in both men and women with schizophrenia and schizoaffective disorder (separately) while taking medication, symptoms, cognition, education, and socioeconomic status into account.

Unlike in SZ and psychosis [51,58,65], nonpsychotic bipolar disorder, and affective disorders, seemed to have comprehension and single-word reading skills comparable to HC [30,47]. Although not all studies specified the type of PD, it seems that reading skill deficits may not be as prominent in nonforensic psychopathy as in SZ.

Effect of diagnosis in forensic samples

Our findings suggest only a weak or no deficit in nonforensic psychopathy but indicate a marked phonological processing and comprehension deficit in the incarcerated group. It is possible that PD/psychopathic individuals with good phonological processing and comprehension are more able to evade incarceration [30,95]. Nonetheless, marked reading deficits in the incarcerated group may have contributed to their poor adjustment within the community [27], which, in turn, increased the risk of incarceration. Men with MIs within forensic settings had significantly lower general reading abilities and spelling than women with MIs [27], consistent with the pattern seen in healthy samples [22].

Clinical implications

Comprehension has a significant influence on decision-making capacity in SZ [96], and this is likely to be true also for people with other MIs, especially within forensic populations. Dyslexia is often underdiagnosed in people with MIs, and this might explain their inability to complete higher education and obtain jobs [15], or the expression of socially unacceptable behaviors [27]. Furthermore, progression and engagement in therapeutic activities within mental health services often depend on good reading and language skills. This highlights a need to accurately identify reading deficits and develop specific programs to improve reading skills of people in psychiatric services. It may be possible to target reading deficits in SZ and other MIs by building on the less affected aspects, such as lexical knowledge (access to words) [97,98], and access to familiar information that can compensate for some of the reading deficits [99], while implementing interventions to ameliorate reading skills [100].

Effect of assessments

Significant between-test differences were found only in tests detecting deficits in comprehension, accuracy, and rate in SZ. In comprehension and rate, the NDRT and GORT-4, and in accuracy, the GORT solely, consistently detected large deficits while the Alouette (French) test detected no deficits (Figure 2). It is conceivable that certain deficits emerge more often/strongly in English compared to some other languages, as is the case in developmental dyslexia [101]. This possibility requires further study.

Promoting a message that highlights threat to life may be effective in raising levels of adherence to measures of infection control, but may also lead to a reduction in health-promoting behaviours

Brown, Richard D., Lynne Coventry, and Gillian V. Pepper. 2020. “COVID-19: The Relationship Between Perceptions of Risk and Behaviours During Lockdown.” OSF Preprints. July 30. doi:10.31219/


Background: Understanding COVID-19 risk perceptions and their impact on behaviour can improve the effectiveness of public health strategies in the future. Prior evidence suggests that, when people perceive uncontrollable risks to their health, they are less likely to make efforts to protect their health in those ways which they can control (e.g. through diet, exercise, and limiting alcohol intake). It is therefore important to understand the extent to which the threat of COVID-19 is perceived to be an uncontrollable risk, and to assess whether this perceived risk is associated with differences in health behaviour.

Methods: We surveyed a nationally representative sample of 496 participants, shortly after the peak of the pandemic in the UK. We collected data to assess people’s perceptions of COVID-19-related risk, and how these perceptions were associated with behaviours. We examined self-reported adherence to behaviours recommended by the UK Government and National Health Service to prevent the spread of the virus, as well as more general health behaviours. We predicted that increased perceived extrinsic mortality risk (the portion of a person’s mortality risk which they perceive to be uncontrollable) would disincentivise healthy behaviour.

Results: Perceived threat to life was found to be the most consistent predictor of reported adherence to measures designed to prevent the spread of infection. Perceived extrinsic mortality risk was found to have increased due to the pandemic, and was also associated with lower reported adherence to Government advice on diet and physical activity, as well as smoking.

Conclusions: Our findings suggest that promoting a message that highlights threat to life may be effective in raising levels of adherence to measures of infection control, but may also have unintended consequences, leading to a reduction in health-promoting behaviours. We suggest that messages that highlight threat to life should be accompanied by statements of efficacy, and that messages evoking feelings of concern for others may also be effective in promoting compliance with anti-infection measures.

No convincing evidence outgroups are denied uniquely human characteristics: Distinguishing intergroup preference from trait-based dehumanization

No convincing evidence outgroups are denied uniquely human characteristics: Distinguishing intergroup preference from trait-based dehumanization. Florence E. Enock et al. Cognition, Volume 212, July 2021, 104682,


• The dual model predicts outgroups are attributed human traits to a lesser extent.

• To date, predominantly desirable traits have been investigated, creating a confound.

• We test attributions of desirable and undesirable human traits to social groups.

• Attributions of undesirable human traits were stronger for outgroups than ingroups.

• We find no support for the predictions of the dual model of dehumanization.

Abstract: According to the dual model, outgroup members can be dehumanized by being thought to possess uniquely and characteristically human traits to a lesser extent than ingroup members. However, previous research on this topic has tended to investigate the attribution of human traits that are socially desirable in nature such as warmth, civility and rationality. As a result, it has not yet been possible to determine whether this form of dehumanization is distinct from intergroup preference and stereotyping. We first establish that participants associate undesirable (e.g., corrupt, jealous) as well as desirable (e.g., open-minded, generous) traits with humans. We then go on to show that participants tend to attribute desirable human traits more strongly to ingroup members but undesirable human traits more strongly to outgroup members. This pattern holds across three different intergroup contexts for which dehumanization effects have previously been reported: political opponents, immigrants and criminals. Taken together, these studies cast doubt on the claim that a trait-based account of representing others as ‘less human’ holds value in the study of intergroup bias.

Keywords: DehumanizationIntergroup biasPrejudiceSocial cognition

7. General discussion

In this paper, we question the central claims of one of the most prominent psychological accounts of dehumanization - the dual model - which holds that outgroup members are perceived as lesser humans than ingroup members by being denied human specific traits (Haslam, 2006). We first revisited work relating to how the lay concept of ‘human’ is best characterised. We then tested its predictions about outgroup dehumanization in a series of seven experiments. Our results present a serious empirical challenge to the dual model.

The dual model argues that there are two sense of humanness: human uniqueness and human nature. Uniquely human traits can be summarised as civility, refinement, moral sensibility, rationality, and maturity. Human nature traits can be summarised as emotional responsiveness, interpersonal warmth, cognitive openness, agency, and depth (Haslam, 2006). However, the traits that supposedly characterise ‘humanness’ within this model are broadly socially desirable (Over, 2020aOver, 2020b). We showed that people also associate some undesirable traits with the concept ‘human’. As well as considering humans to be refined and cultured, people also consider humans to be corrupt, selfish and cruel.

Results from our pretest provided us with grounds for re-examining predictions made by the dual model of dehumanization about the nature of intergroup bias in trait attributions. The dual model account holds that lesser attribution of human specific traits to outgroup members represents a psychological process of dehumanization that is separable from ingroup preference. However, as the human specific attributes summarised by the model are positive and socially desirable, it is possible that previous findings are better explained in terms of ingroup preference, the process of attributing positive qualities to ingroup members to a greater extent than to outgroup members.

In seven highly-powered experiments, we tested the predictions of the dual model against this alternative. We pitted the two hypotheses against each other by comparing attributions of uniquely human traits that varied in whether they were socially desirable or undesirable to ingroup and outgroup members. The dual model holds that subtle dehumanization is evidenced by denying outgroup members uniquely human traits relative to ingroup members. We reasoned that whereas outgroup members may be denied desirable human traits, they are likely to be attributed undesirable human traits to a greater extent than ingroup members.

Across three distinct intergroup contexts, we found no evidence for either animalistic or mechanistic dehumanization of outgroup members. Instead, we found strong and reliable intergroup preference effects. Desirable traits were ascribed more strongly to ingroup members than outgroup members and undesirable traits more strongly to outgroup members than ingroup members, irrespective of perceived humanness.

A possible defence of the dual model account could be to argue that we chose three intergroup contexts in which animalistic and mechanistic dehumanization does not occur. However, we chose to investigate judgements of political opponents, immigrants and criminals specifically because previous research has suggested that they are dehumanized on a range of measures (Banton et al., 2020Bastian et al., 2013Markowitz & Slovic, 2020Pacilli et al., 2016Viki et al., 2013). In addition, we also showed in every experiment that outgroup members were explicitly rated as less human than were the ingroup on the blatant dehumanization scale (Kteily et al., 2015). Prior work shows that measures of animalistic and mechanistic dehumanization correlate positively with blatant dehumanization scores (Kteily et al., 2015). Though they are not claimed to measure the same construct, they have been shown to reliably co-occur. These findings confirm that these are the sorts of intergroup contexts in which we would expect to see trait-based dehumanization should the process occur.

We acknowledge that without testing all possible intergroup contexts, it remains a possibility that some outgroups could be denied human specific attributes relative to ingroups even when valence is appropriately controlled for. In other words, it could be the case that trait-based dehumanization occurs independently of ingroup preference in some social settings. It may be particularly interesting for future research to investigate intergroup contexts that are not so strongly associated with competition, threat and animosity.

However, the possibility that some, as yet untested, groups may be denied human unique attributes does not detract from the importance of our critique. To accurately measure trait-based dehumanization in future research, studies must consider the central role of valence. Prior work utilising the dual model framework has reported dehumanization to be extremely widespread in society, affecting not just marginalised groups but doctors, patients and even cyclists (Delbosc, Naznin, Haslam, & Haworth, 2019Haslam & Stratemeyer, 2016). Rigorous measurement and tighter experimental control may change some or all of the conclusions from previous research.

Across our experiments, we observed strong intergroup preference effects, with desirable traits more strongly ascribed to the ingroup and undesirable traits more strongly to the outgroup. Our results demonstrate both ingroup favouritism (assigning greater positivity to the ingroup) and outgroup derogation (assigning greater negativity to the outgroup) (Brewer, 1999Hewstone et al., 2002). However, we also suggest that group specific stereotypes are likely to play an important role in these processes. In many social contexts, trait attributions may reflect social stereotyping as well as intergroup preferences (Fiske et al., 2002). For example, previous work suggested that Anglo-Australians were ‘animalistically’ dehumanized both by themselves and by Ethnic-Chinese participants, whilst Ethnic-Chinese people were ‘mechanistically’ dehumanized both by themselves and by Anglo-Australians (Bain et al., 2009). These effects may be more compatible with stereotype content than with trait-based dehumanization. Future work would benefit from addressing the distinction between stereotyping and trait-based dehumanization.

An outstanding question relates to whether other psychological models of dehumanization more accurately capture the ways in which different social groups are perceived. For example, infrahumanisation theory predicts that people tend to believe ingroup members experience uniquely human emotions more strongly than do outgroup members (Leyens et al., 2000Leyens et al., 2001). It would be valuable for future research to examine the utility of this theory by testing whether participants perceive ingroup members to experience human emotions more strongly overall or whether they perceive ingroup members to experience prosocial emotions more strongly but outgroup members to experience antisocial emotions more strongly. Further work could helpfully investigate how these findings bare on the claim that outgroups are sometimes dehumanized by being denied mental states (Harris & Fiske, 2006).

Taken together, our studies suggest that the dual model does not accurately characterise the ways in which outgroups are perceived in at least the social contexts examined here – political groups, immigrants and criminals. Prejudice and discrimination are pressing social problems. If psychological research is to contribute to the interdisciplinary mission to reduce prejudice and encourage more egalitarian behaviour, then it must start by accurately characterising the psychological biases underlying discriminatory behaviour. We suggest that the dual model of dehumanization conflates apparent evidence for dehumanization with ingroup preference. As a result, it may obscure more than it reveals about the psychology of intergroup bias.

Women were much more likely to report long-term mate attraction to the benevolent, vs to the hostile, sexist; the hostile sexist one generated greater distrust and dislike in both men & women, compared to the benevolent one

Grab 'em by the...: Hostile and benevolent sexism as signals of mating strategies. Adair, L. & Ferenczi, N. European Human Behaviour and Evolution Association, 15th Conference, Mar 2021.


Objective: Men’s hostile sexism is associated with many negative interpersonal consequences (e.g., lower relationship quality). Men’s benevolent sexism does not seem to similarly promote negative behaviours and interpersonal interactions but is still associated with harmful attitudes and stereotypes (e.g., endorsement of rape myths). Recent work finds that disclosures of benevolent sexism are associated with perceived attractiveness and provisioning qualities, even though they are perceived as patronising. Our work is designed to apply evolutionary and feminist perspectives to investigate this further – is sexism used to infer mating-relevant qualities, mate attractiveness, and mating strategy?

Methods: Using a nationally representative sample (N = 317; Female = 50%), participants were randomly assigned to evaluate descriptions of men varying along two criteria, sexism (hostile vs. benevolent) and social prestige (with vs. without prestige criteria).

Results: 2x2 Between-subjects ANOVAs highlighted several main effects: benevolent sexists were rated higher on long-term mate qualities, hostile sexists were rated higher on short-term mate qualities, and hostile sexists were rated as “more sexist” overall. Interaction effects indicated that having prestige decreased how “sexist” hostile sexists were perceived, the same was not true for benevolent sexists.

Conclusions: As predicted, disclosures of benevolent sexism were used to infer long-term mating qualities, while disclosures of hostile sexism were used to infer short-term mating qualities. We found that women were much more likely to report long-term mate attraction to the benevolent, compared to the hostile, sexist. The hostile sexist generated greater distrust and dislike in both men and women, compared to the benevolent sexist.

The trophic level of the Homo lineage that most probably led to modern humans evolved from a low base to a high, carnivorous position during the Pleistocene; partially reversed later & with the advent of agriculture

The evolution of the human trophic level during the Pleistocene. Miki Ben‐Dor  Raphael Sirtoli  Ran Barkai. American Journal of Physical Anthropology, March 5 2021.

Abstract: The human trophic level (HTL) during the Pleistocene and its degree of variability serve, explicitly or tacitly, as the basis of many explanations for human evolution, behavior, and culture. Previous attempts to reconstruct the HTL have relied heavily on an analogy with recent hunter‐gatherer groups' diets. In addition to technological differences, recent findings of substantial ecological differences between the Pleistocene and the Anthropocene cast doubt regarding that analogy's validity. Surprisingly little systematic evolution‐guided evidence served to reconstruct HTL. Here, we reconstruct the HTL during the Pleistocene by reviewing evidence for the impact of the HTL on the biological, ecological, and behavioral systems derived from various existing studies. We adapt a paleobiological and paleoecological approach, including evidence from human physiology and genetics, archaeology, paleontology, and zoology, and identified 25 sources of evidence in total. The evidence shows that the trophic level of the Homo lineage that most probably led to modern humans evolved from a low base to a high, carnivorous position during the Pleistocene, beginning with Homo habilis and peaking in Homo erectus. A reversal of that trend appears in the Upper Paleolithic, strengthening in the Mesolithic/Epipaleolithic and Neolithic, and culminating with the advent of agriculture. We conclude that it is possible to reach a credible reconstruction of the HTL without relying on a simple analogy with recent hunter‐gatherers' diets. The memory of an adaptation to a trophic level that is embedded in modern humans' biology in the form of genetics, metabolism, and morphology is a fruitful line of investigation of past HTLs, whose potential we have only started to explore.


7.1 Summary of the evidence

The actual HTL during the Pleistocene is unobservable; therefore, we looked for evidence of the impact of the HTL on the studied humans' biological, behavioral, and ecological systems. Observing only its reflection and not the HTL itself, we are left employing varying degrees of interpretation in forming an opinion about what HTL caused a specific impact and whether it denotes a step toward specialization or generalization. We reviewed 25 different evidence sources, 15 of which are biological and the remainder archaeological, paleontological, zoological, and ethnographic. With a list of 25 evidence items, by necessity, the descriptions of the findings and the justification of the various interpretations cannot be thorough, leaving much room for further review and debate. One of the main aims of this article was to present the power of paleobiology in particular and to cast a wide net of scientific fields in general in investigating HTL. The evidence here is by no means comprehensive. There is additional genetic and physiological evidence for biological adaptations to a higher and lower trophic level in the past and additional evidence for Paleolithic HTLs in other fields of science, which we chose not to include, and others that we probably missed. Thus, although we do not shy away from presenting a clear hypothesis based on the evidence, the article should be looked at as a first iteration in what hopefully will be a continuous scientific endeavor.

The presented evidence can be organized into three epistemological groups with potentially increasing levels of validity regarding a specific trophic level (see text or Table 2 for a description of the evidence):

TABLE 2. Paleolithic human trophic level: Summary of results (summary of the evidence)
BioenergeticsHumans had high energetic requirements for a given body mass and had a shorter time during the day to acquire and consume food. Hunting provides tenfold higher energetic return per hour compared to plants, leaving little room for flexibility (plasticity) between the two dietary components. Animals tend to specialize in the highest return items in their niche. (Specialization)
Diet quality

In primates, a larger brain is associated with high energy density food. With the largest brain among primates, humans are likely to have targeted the highest density food, animal fats, and proteins.

Brain size declined during the terminal Pleistocene, and subsequent Holocene. Diet quality declined at the same time with the increased consumption of plants. (Specialization)

Higher fat reservesWith much higher body fat reserves than primates, humans are uniquely adapted to lengthy fasting. This adaptation may have helped with overcoming the erratic encountering of large prey. (Change in trophic level) (Change in trophic level)
Genetic and metabolic adaptation to a high‐fat dietHumans adapted to higher fat diets, presumably from animals. Study (Swain‐Lenz et al., 2019): “suggests that humans shut down regions of the genome to accommodate a high‐fat diet while chimpanzees open regions of the genome to accommodate a high sugar diet” (Change in trophic level)
FADS‐Omega3 oils metabolismGenetic changes in the FADS gene in African humans 85 Kya allowed for a slight increase in the conversion of the plant DHA precursor to DHA signaling adaptation to a higher plant diet (Change in trophic level)
Late adaptations to tubers' consumptionTubers were assumed to be a mainstay of Paleolithic diets that cooking could prepare for consumption. Recent groups that consume high quantities of tubers have specific genetic adaptations to deal with toxins and antinutrients in tubers. Other humans are not well adapted to consume large quantities of tubers. (Change in trophic level)
Stomach acidityHigher stomach acidity is found in carnivores to fight meat‐borne pathogens. Humans' stomach acidity is even higher than in carnivores, equaling that of scavengers. Adaptation may have evolved to allow large animals' consumption in a central place over days and weeks with pathogen build‐up. (Categorization to a trophic group)
Insulin resistanceHumans, like carnivores, have a low physiological (non‐pathological) insulin sensitivity.
Gut morphologyHumans' gut morphology and relative size are radically different from chimpanzees' gut. Longer small intestines and shorter large intestines are typical of carnivores' gut morphology and limit humans' ability to extract energy from plants' fiber. (Specialization)
MasticationReduction of the masticatory system size already in Homo erectus, compared to early hominins, who relied on terrestrial vegetation as a food source. The reduced size is compatible with meat and fat consumption. (Aiello & Wheeler, 1995; Zink & Lieberman, 2016) (Change in trophic level)
CookingCooking was hypothesized to have enabled plants' high consumption despite the need for a high‐quality diet, starting with H. erectus. Other researchers argue that habitual use of fire is evident only around 0.4 Mya. Also, a fire has other uses and is costly to maintain. (Change in trophic level)
Postcranial morphologyA set of adaptations for endurance running is found already in H. erectus, useful in hunting. Shoulders adapted to spear‐throwing in H. erectus. But limited tree climbing capability. (Specialization)
Adipocyte morphologySimilar to the morphology in carnivores. “These figures suggest that the energy metabolism of humans is adapted to a diet in which lipids and proteins rather than carbohydrates, make a major contribution to the energy supply.” (Pond & Mattacks, 1985). (Categorization to a trophic group)
Age at weaningCarnivores wean at a younger age, as do humans. Early weaning “highlight the emergence of carnivory as a process fundamentally determining human evolution” (Psouni et al., 2012). (Categorization to a trophic group)
LongevityKaplan et al. (2007) hypothesized that a large part of the group depended on experienced hunters due to long childhood. Extended longevity in humans evolved to allow utilization of hunting proficiency, which peaks by age 40. The grandmother hypothesis claim women's longevity allowed additional gathering. (Change in trophic level)
VitaminsHypothesis for required nutritional diversity to supply vitamins is contested. It appears that all vitamins, including vitamin C are supplied in adequate quantities on a carnivorous diet. (Neutral)
Multicopy AMY1 genesMulti‐copies of the AMY1 gene have been hypothesized as adaptive to high starch diets. However, both findings of its possible lack of functionality and the unclear timing of its appearance limits the use of the evidence to support a change in trophic level. (Change in trophic level)
PlantsPlants were consumed throughout the Paleolithic, but their relative dietary contribution is difficult to assess. Recent advances in plant residues identification in dental pluck provide non‐quantitative evidence of widespread plants' consumption. Division of labor may point to a background level of plant supply, but the evidence is based largely on ethnography, which may not be analogous to the Pleistocene. (Inconclusive)
Stone toolsStone tools specific to plant food utilization appear only some 40 Kya, and their prevalence increases just before the appearance of agriculture, signaling increased plant consumption toward the end of the Paleolithic. (Change in trophic level)
ZooarchaeologyFirst access to large prey, denoting hunting, appears already in H. erectus archaeological sites 1.5 Mya. Humans also hunted large carnivores. (Change in trophic level)
Targeting fatHumans concentrated on hunting fatty animals at substantial energetic costs. They preferentially brought fatty parts to base camps, hunted fattier prime adults, and exploited bone fat. That behavior may indicate that plants could not have been easily obtained to complete constrained protein consumption. (Specialization)
Stable IsotopesNitrogen 15N isotope measurement of human fossil collagen residues is the most extensively used method for determining trophic level in the last 50 thousand years. All studies show that humans were highly carnivorous until very late, before the appearance of agriculture. The method has some shortcomings but was able to identify variation in trophic level among present‐day traditional groups. (Categorization to a trophic group)
Dental pathologyDental caries, evidence of substantial consumption of carbohydrates, appeared only some 15 Kya in groups with evidence of high plant food consumption. (Change in trophic level)
Dental wearDifferent wear on fossilized teeth as a result of different diets has the potential to reconstruct diets. However, the claim for the reconstruction of variable diets in Paleolithic humans could not be verified as the comparison the groups' diets were unclear. (Inconclusive)
Behavioral adaptationsA comparison of humans' behavior patterns with chimpanzees and social carnivores found that humans have carnivore‐like behavior patterns. Food sharing, alloparenting, labor division, and social flexibility are among the shared carnivorous behaviors. (Categorization to a trophic group)
Paleontological evidence

Evidence for hunting by H. erectus 1.5 Mya was associated with the extinction of several large carnivores, but not smaller carnivores. This suggests that H. erectus became a member of the large carnivores' guild. Extinction of large carnivores in Europe also coincided with the arrival of humans (Categorization to a trophic group)

Extinctions of large herbivores were associated with humans' presence in Africa and arrival to continents and islands, such as Australia and America, suggesting preferential hunting of large prey.

Zoological analogyHumans were predators of large prey. In carnivores, predation on large prey is exclusively associated with hypercarnivory, i.e., consuming over 70% of the diet from animals. (Categorization to a trophic group)
EthnographyVariability in trophic level in the ethnographic context is frequently mentioned as proof of HTL variability during the Paleolithic. However, ecological and technological considerations limit the analogy to the terminal Paleolithic. (Change in trophic level)

7.2 Type 1. Change in trophic level

  • Higher fat reserves
  • Adaptations to a high‐fat diet
  • Late adaptation to USO
  • FADS—omega 3 metabolism
  • Amy1 multicopy
  • Longevity—Fathers
  • Longevity—Mothers
  • Dental pathology
  • Cooking
  • Zooarchaeology
  • Stone tools
  • Ethnography

This group includes evidence for physiological or behavioral adaptations to acquire and consume higher or lower amounts of either animal or plant‐sourced food. There is no argument that the evolution of the genus Homo was associated with increasing HTLs in H. habilis and further in H. erectus; therefore, it is not surprising that most of this type of evidence in the Lower Paleolithic includes evidence for adaptation to carnivory. Detailing these pieces of evidence may thus appear to be superfluous; however, identifying a trend in the number and timing of the acquisition of the adaptation may supply important indications for the occurrence of a change in trophic level. Accumulation of type 1 evidence in the Late UP supports a significant change to lower HTL at that period. Also, evidence in one period and not in the other can be interpreted as evidence for a different HTL in the last. For example, if we accept that the appearance of the AMY 1 gene multicopy sometime in H. sapiens evolution suggests a higher consumption of starch, we have to accept that there was no pressure to adapt in prior species to high consumption of starch. The same logic applies to the appearance of grains handling stone tools and dental pathology that appear only toward the Paleolithic end.

7.3 Type 2. Specialization—Reduced flexibility

  • Bioenergetics
  • Diet quality
  • Gut morphology
  • Mastication
  • Postcranial morphology
  • Targeting fat

Since we cannot observe our subjects, evidence for specialization is defined here as evidence that is similar to Type 1 but that, at the same time, potentially reducing the phenotypic plasticity of humans by hindering the acquisition or assimilation of the other food group (plant or animal).

Specialization and generalization must be defined with reference to particular axes such as temperature, habitat, and feeding (Futuyma & Moreno, 1988). Pineda‐Munoz and Alroy (2014) defined feeding specialization as selecting 50% or more of the food from a certain food group (fruits, seeds, green plants, insects, or vertebrates). By this definition, humans could be called specialists if they selected to consume 50% or more of their diet from vertebrates or another group of plants or invertebrates.

Another axis on which specialization can be defined is prey size. Large carnivores specialize in large prey. Evidence for humans' specialization in large herbivores can contribute to the continuing debate regarding humans' role in megafauna extinction (Faith et al., 2020; Faurby et al., 2020; Sandom et al., 2014; F. A. Smith et al., 2018; Werdelin & Lewis, 2013) and the implications of megafauna extinction on humans. Potts et al. (2018) identified an association between prey size decline during the Middle Pleistocene and the appearance of the MSA, and Ben‐Dor et al. (2011) further proposed that the extinction of elephants in the Southern Levant led to the appearance of the Acheulo‐Yabrudian cultural complex 420 Kya. Ben‐Dor and Barkai (2020a) have argued that humans preferred to acquire large prey and that large prey is underrepresented in zooarchaeological assemblages (Ben‐Dor & Barkai, 2020b). Listed in Table 3 is evidence, among the ones collected here, that can be interpreted as supporting humans' specialization in acquiring large prey.

TABLE 3. Specialization in the acquisition of large prey
BioenergeticLarge prey provides higher energetic returns than smaller prey. The need to substitute large prey for smaller prey is energetically costly.
Higher fat reservesLarge prey is less abundant than smaller prey. Fat reserves may have evolved to allow extended fasting of several weeks, thereby bridging an erratic encountering rate with large prey. Humans have adapted to easily synthesize ketones to replace glucose as an energy source for the brain.
Stomach acidityStronger acidity than carnivores' can be interpreted as an adaptation to a large prey's protracted consumption over days and weeks, whereby humans are acting as scavengers of their prey.
Targeting fatThe recognition of targeting fat as a driver of human behavior supports the importance of large, higher fat bearing animals to humans' survival.
Stable isotopesHigher levels of nitrogen isotope 15 than carnivores were interpreted by researchers as testifying to the higher consumption of large prey than other carnivores.
PaleontologyA decline in the guild of large prey carnivores 1.5 Mya is interpreted as resulting from humans' entrance to the guild. Also, the extinction of large prey throughout the Pleistocene is interpreted by some researchers as anthropogenic, testifying to humans' preference for large prey.
Zoological analogyLarge social carnivores hunt large prey.
EthnographyInterpreting ethnographic and Upper Paleolithic technologies as an adaptation to the acquisition of smaller prey means that humans were less adapted to the acquisition of smaller prey in earlier periods.

Dietary plasticity is a function of phenotypic plasticity (Fox et al., 2019) and technological and ecological factors. In many cases, evolution is a process of solving problems with trade‐offs (Garland, 2014). Identifying features that were traded‐off in specific adaptations could inform us of changing dietary phenotypic plasticity levels. Relative energetic returns on primary (plant) and secondary (animal) producers are key to assessing plasticity's ecological potential. In humans, technology can expend plasticity by enabling and increasing the energetic return on the acquisition of certain food items. Bows for the hunting of smaller, faster prey and grinding stones are two examples of such technologies.

We mainly listed specialization adaptations that affected phenotypic plasticity, but there are technological and ecological pieces of evidence that potentially changed dietary plasticity like the invention of bows that increased the level of plasticity regarding prey size and the appearance and disappearance of savannas with the accompanied change in primary to secondary production ratios that swayed plasticity toward primary or secondary producers.

7.4 Type 3. Categorization to a trophic group

  • Stomach acidity
  • Adipocyte morphology
  • Age at weaning
  • Stable isotopes
  • Behavioral adaptations
  • Paleontologic evidence
  • Zoological analogy
  • Insulin resistance

All the eight pieces of evidence of membership in a trophic group concluded that humans were carnivores. Assigning humans to a specific dietary trophic group has the highest potential validity, as it answers the research question with minimal interpretation.

In some cases, interpretation is required to assign a phenomenon to HTL. Belonging to the carnivores' trophic groups still does not tell us if humans were 90% or 50% carnivores. It does tell us, however, that humans were carnivorous enough and carnivorous long enough to justify physiological and behavioral adaptations unique to carnivores. Following the zoological analogy with large social carnivores that acquire large prey, we hypothesized that humans were hypercarnivores, defined as consuming more than 70% of the diet from animal sources.