human diets, particularly the roots, fruits,and nuts of plants. Plant materials areused to construct a diverse array of arti-

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can be found online at https://doi.org/10.1016/j.tics.

2019.04.010.

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How Plants Shapethe Mind

facts and shelters. Plant chemicals areused to facilitate hunting and fishing, aswell as in rituals and medicines. However,despite all of these benefits, plants can in-flict serious costs. Plants have evolved animpressive set of defenses to protectagainst damage from herbivores [2]. Allplants produce toxic chemical defenses,

1Brain and Cognitive Sciences Department, MassachusettsInstitute of Technology, Cambridge, MA, USA2McGovern Insitute for Brain Research, Massachusetts Instituteof Technology, Cambridge, MA, USA3Psychiatry Department, Massachusetts General Hospital, Boston,MA, USA4Eliot-Pearson Deparmtent of Child Study and HumanDevelopment, Tufts University, Medford, MA, USA

Annie E. Wertz1,*

Plants are easy to overlook in mod-ern environments, but were a fun-damental part of human life overevolutionary time. Recent workwith infants suggests that theadaptive problems humans facedwith respect to plants have lefttheir mark on the human mind.

Plants Are A(n Adaptive) ProblemIn many societies, plants are no longer aconspicuous part of human life. Plantsare a part of the scenery outside and avail-able for purchase, already packaged andprocessed, in grocery stores and gardencenters. This limited contact with plantsmay seem perfectly normal, but acrossthe entirety of human history it is quite un-usual. Taking as a starting point the emer-gence of the genus Homo, humans spent99% of our evolutionary history ashunter–gatherers. In a hunter–gathererworld, there were no such shops wherethe necessities of life could be easily ac-quired. Instead, our ancestors had tomake a living by effectively utilizing the nat-ural environment. Plants were an essentialpart of this process.

The archeological record and studies ofmodern hunter–gatherer and hunter–horti-culturalist populations show that humansrelied on plants in a variety of ways [1].Plants are an important component of

some of which can be harmful or evenfatal to humans when ingested. Someplants also have mechanical defenses,such as thorns or stinging hairs, that cancause serious skin injury and in somecases systemic effects.

The problem is: how do humans figure outwhich plants are food (or otherwise useful)and which ones are fatal? This turns out tobe a very difficult task. There are myriadplant species and herbivores that feed onthem. The result of these complex coevo-lutionary relationships is that, from ahuman perspective, there are no morpho-logical features common to all edible ortoxic plants, even in the scale of the envi-ronments humans typically encounterwithout modern means of travel. Therefore,using general rules such as ‘Avoid plantswith white flowers’ or ‘Purple fruits are edi-ble’ simply would not work. In the formercase, one would miss out on pears, andin the latter one would end up eating deadlynightshade. Importantly, the presence ofdifficult-to-detect and potentially fatal toxinsmakes learning about plants through trialand error sampling very costly. The bestoutcome for this process involves largeamounts of wasted time and repeatedexposure to noxious plant defenses. Theworst-case scenario is death. These kindsof circumstances select for the evolutionof social learning mechanisms [3].

The recurrent adaptive problems our spe-cies encountered over evolutionary timehave shaped the human mind [4] and it iswell known that plant defenses have struc-tured the physiology and behavior of many

*Correspondence:evelina9@mit.edu (E. Fedorenko) andMarina.Bers@tufts.edu (M.U. Bers).@1Twitter: @ev_fedorenko (E. Fedorenko) and @marinabers(M.U. Bers).

https://doi.org/10.1016/j.tics.2019.04.010

© 2019 Elsevier Ltd. All rights reserved.

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3. Nakagawa, T. et al. (2014) Quantifying programmers’mental workload during program comprehensionbased on cerebral blood flow measurement: a con-trolled experiment. In Companion Proceedings of the36th International Conference on Software Engineering(ICSE2014), pp. 448–451, ACM

4. Shneiderman, B. and Mayer, R.E. (1975) Towards a Cogni-tive Model of Programmer Behavior, Indiana University

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(C)

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Figure 1. Testing Plant Learning and Avoidance of Natural Toxins (PLANT). (A) Some of the stimulus items used in our studies testing infants’ behavioralavoidance of plants. The stimulus types include real plants, artificial plants that do not have a scent or change during the course of testing, novel artifacts matched toshape and/or color features of the plants, familiar artifacts some of which infants are typically allowed to handle themselves and others that are typically prohibited, and naturalobjects like shells and stones. (B) The experimental setup. The experimenter places each stimulus object in front of the infant in a counterbalanced order and maintains aneutral expression. Parents’ eyes remain closed throughout the testing session to preclude their reactions to the stimuli influencing their child’s behavior. Infants’ reachingand looking behavior are later coded from videos of the session. (C) Representative touch latency results from one of our studies. Adapted, with permission, from [11].As predicted, infants take longer to reach out and touch plants (both real and artificial) than all of the other object types. We found a similar pattern of results in [7,10].

animal species [2], including humans [5].Accordingly, along with my colleagues, Ihave recently proposed a solution for thelearning problems plants pose. We arguethat the human mind contains a collectionof behavioral avoidance strategies and so-cial learning rules geared toward safely ac-quiring information about plants [6,7]. I willrefer to this collection of cognitive systemsas Plant Learning and Avoidance of Natu-ral Toxins, or PLANT.

Evidence for PLANTWe have begun testing PLANT with stud-ies of human infants. One line of work ex-amines whether infants possessbehavioral strategies that would mitigateplant dangers, similar to plant food rejec-tions in older children [8]. Unlike the ani-mate dangers that infants readily attend

to (e.g., snakes, spiders [9]), dangerousplant toxins are difficult to detect but rela-tively easy to avoid. Plants are quite literallyrooted to the spot and consequently caninflict harm only on individuals that comeinto contact with them. Therefore, we pro-pose that PLANT includes behavioralavoidance strategies that protect infantsby minimizing their physical contact withplants. To test this proposal, we presentinfants with plants and different kinds ofcontrol objects and measure their reachingbehavior (Figure 1). Our results show that,as predicted, infants are reluctant to touchplants compared with other object types[7,10,11] and touch plants less frequentlyafter making contact with them [10]. Infantsare similarly avoidant of benign-lookingplants and plants covered in sharp-lookingthorns [10], suggesting that they initially

treat all plants as potentially dangerous – asensible strategy given that delicate-looking plants can be deadly poisonous.

Of course, not all plants can be avoided.Plant foods and materials must be for-aged, which necessarily means cominginto contact with plants. In some modernhunter–gatherer societies, plant foragingcan begin as early as 2–3 years of age[1]. Therefore, in a second line of work,we are investigating whether infants arevigilant for social information about plantsand use it to guide their behavior. Thesestudies allow us to test the proposal thatPLANT includes social learning rules.Thus far, we have found that infants lookmore often to adults when they first en-counter plants, in the time before touch-ing them [11], suggesting that behavioral

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Our laboratory studies are currently examining, among otherareas, (i) plant food learning in infancy, (ii) the structure of sociallearning and generalization rules for danger and edibilityinformation, and (iii) visual features infants use to distinguishplants from other entities.

What is the underlying structure of PLANT?(A) How does PLANT operate in naturalistic settings?(B)

How does PLANT operate in other species?

Humans are hardly unique in our reliance on plants overevolutionary time. Studying the behavior of other species canhelp us understand how different evolutionary histories andnatural ecologies impact plant-related cognitive systems (imagecredit: Carmen Kaiser).

(D)

Children and adults across the world have very differentexperiences with plants. Of particular interest are societieswhere individuals have frequent contact with plants andextensive ecological knowledge (Achuar children; image credit:Ulises Espinoza).

How is PLANT affected by different cultural contexts?(C)

We are also currently observing how infants and young childrenlearn from others in outdoor garden settings. These studiesprovide an important real-world complement to the studies weconduct in our lab (image credit: Arne Sattler/MPIB).

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Figure 2. Open Questions and Future Directions for Plant Learning and Avoidance of Natural Toxins (PLANT). These lines of inquiry include (A) laboratoryresearch, (B) naturalistic observations, (C) cross-cultural investigations, and (D) comparative studies.

avoidance strategies operate in concertwith social learning processes. Thisstructure would enable infants toobserve signals from adults before mak-ing contact with potentially dangerousplants. Infants appear to be particularlyattuned to social signals that allow themto learn which plants are edible [6,12]. Inour studies, we show 6- and 18-month-olds an adult eating pieces of fruit from aplant and a manmade object (Figure 2A).Despite seeing the same social informa-tion demonstrated with both objecttypes, infants identify the plant, over

the artifact, as a food source [6]. Onceinfants have learned that fruits from aparticular plant are edible, 18-month-olds generalize this information only toother plants that share the same leafshape and fruit color [12]. This combina-tion of social learning and restrictivegeneralization rules would prevent in-fants from inadvertently ingesting toxicplants.

Seeing the Forest for the TreesOur empirical findings to date are consis-tent with the proposed PLANT systems.

Infants appear to deploy a collection of be-havioral avoidance strategies and sociallearning rules for plants. Consequently,PLANT minimizes infants’ exposure toharmful plant defenses and allows themto safely acquire information about thespecific plants they encounter from moreknowledgeable individuals. In short, thiswork supports the claim that plants haveshaped the human mind.

PLANT is a novel research area that pro-vides fertile ground for future exploration(Figure 2). Of particular interest are cross-

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cultural studies that can shed light on the de-velopment of PLANT in different environ-ments and comparative studies that canclarify the evolution of PLANT. Further, theintegral role that plants played in human lifeand human evolution means that PLANT islikely to be enmeshed in a web of cognitivesystems that support broader capacitieslike food learning, threat mitigation, categori-zation, and cultural transmission, amongothers. This interconnectedness makesPLANT an excellent starting point for futureinquiry in these areas. At the same time, itis highly unlikely that infants, or adults forthat matter, will treat plants as a special cat-egory in all circumstances. Research oncognitive systems like PLANT can providenew ways of exploring fundamental aspectsof human cognition and understanding theevolution of learning.

AcknowledgmentsThis work was funded by the Max Planck Society.

1Max Planck Institute for Human Development, Max PlanckResearch Group Naturalistic Social Cognition, Lentzeallee 94,14195 Berlin, Germany

*Correspondence:wertz@mpib-berlin.mpg.de (A.E. Wertz).

https://doi.org/10.1016/j.tics.2019.04.009

© 2019 Elsevier Ltd. All rights reserved.

References1. Hardy, K. and Martens, L.K., eds (2016)Wild Harvest: Plants

in the Hominin and Pre-agrarian Human Worlds, OxbowBooks

2. Mithöfer, A. and Boland, W. (2012) Plant defense againstherbivores: chemical aspects. Annu. Rev. Plant Biol. 63,431–450

3. Perreault, C. et al. (2012) A Bayesian approach to theevolution of social learning. Evol. Hum. Behav. 33,449–459

4. Barrett, H.C. (2015) The Shape of Thought, Oxford Uni-versity Press

5. Placek, C.D. and Hagen, E.H. (2015) Fetal protection: theroles of social learning and innate food aversions in SouthIndia. Hum. Nat. 26, 255–276

6. Wertz, A.E. and Wynn, K. (2014) Selective social learningof plant edibility in 6- and 18-month-old infants. Psychol.Sci. 24, 874–882

7. Wertz, A.E. and Wynn, K. (2014) Thyme to touch: infantspossess strategies that protect them from dangers posedby plants. Cognition 130, 44–49

8. Lafraire, J. et al. (2016) Food rejections in children: cogni-tive and social/environmental factors involved in foodneophobia and picky/fussy eating behavior. Appetite 96,347–357

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11. Elsner, C. and Wertz, A.E. (2019) The seeds of sociallearning: infants exhibit more social looking for plantsthan other object types. Cognition 183, 244–255

12. Wertz, A.E. and Wynn, K. (2019) Can I eat that too? 18-month-olds generalize social information about edibilityto similar looking plants. Appetite 138, 127–135

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Rethinking CognitiveLoad: A Default-ModeNetwork PerspectiveAdrianna C. Jenkins 1,*

Typical cognitive load tasks arenow known to deactivate thebrain’s default-mode network(DMN). This raises the possibilitythat apparent effects of cognitiveload could arise from disruptionsof DMN processes, includingsocial cognition. Cognitive loadstudies are reconsidered, withreinterpretations of past researchand implications for dual-processtheory.

IntroductionResearch in the mind sciences often usescognitive load to distinguish automaticfrom controlled mental processes. In thepast 5 years, more than 1000 journalarticles have included the keyword ‘cogni-tive load’ (https://www.ncbi.nlm.nih.gov/pubmed). During that time, experimentsusing cognitive load tasks (see Glossary)have informed conclusions about theautomatic and controlled componentsof nearly every aspect of humanthought and behavior, ranging fromvisual perception to stereotyping, socialcomparison, and even juror sentencingdecisions [1]. Such experiments havebeen particularly influential in the domainof social thought and behavior, where

they have contributed to debates abouthuman nature, including the automaticityof human prosociality.

A fundamental assumption behind typicalcognitive load experiments is that thereexists a distinction between two sets ofprocesses in the mind. On such dual-process accounts of cognition, one set ofprocesses is thought to be relatively fast,automatic, and/or intuitive (‘System 1’),whereas another is thought to be relativelyslow, controlled, and/or deliberative (‘Sys-tem 2’). In a typical experiment, partici-pants perform a cognitive load taskthought to ‘occupy’ the resources of Sys-tem 2 (e.g., mental arithmetic, digit stringmemorization), thereby diminishing theavailability of those resources for a concur-rent task (Figure 1).

Although tremendously influential, the dis-tinction between System 1 and System 2has attracted increasing scrutiny in recentyears [2–4]. The origins of this scrutinyrange from inconsistent results in cognitiveload experiments to evidence that a dis-tinction between System 1 and System 2does not necessarily map onto the func-tional organization of the brain, leadingsome commentators to suggest thatdual-process theories should be aban-doned altogether. Here, I offer a perspec-tive that, first, has the potential to clarifysome of the mixed results from cognitiveload experiments and, second, offers away to reinterpret many cognitive load ef-fects without relying on dual-processassumptions.

The DMNOne of the most striking findings incognitive neuroscience has been the iden-tification of the DMN. The brain regions ofthe DMN have a distinctive functionalprofile, characterized by higher activitythan other regions of the brain at baseline(i.e., when people are not engaged ina particular task) and deactivationswhen people direct their attention to a

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