Professor Andy Wills
Profiles

Professor Andy Wills

Professor in Psychology

School of Psychology (Faculty of Health)

Professor Andy Wills can be contacted through arrangement with our Press Office, to speak to the media on these areas of expertise.
  • Psychology
  • Neuroscience
  • Artificial intelligence
  • Machine learning
  • Computational neuroscience
  • Cognitive science
  • Bayesian statistics
  • Big Data
  • Experimental methods
  • Neuroimaging
  • Quantitative research
  • Systematic reviews
Biography

Biography

Professor of Psychology
Personal website: www.andywills.info

Qualifications

B.Sc. Psychology (Soton), Ph.D. Psychology (Cantab)

Academic background

I became interested in artificial intelligence through an early interest in computers (I was paid for my first computer program at age 14). I studied psychology as an undergraduate (Southampton, 1990 - 1993) because of a (still-held) belief that psychology can inform the design of intelligent systems. A continuing interest in neural networks led to me studying for a Ph.D. with computational learning theorist Ian McLaren (Cambridge, 1994 - 1998). I continued my Ph.D. research as a Junior Research Fellow (Emmanuel College, Cambridge, 1998 - 2000).

Keen to secure a permanent position, and interested in teaching as well as research, I moved to Exeter University, where I became a lecturer (2000), senior lecturer (2005), and then associate professor (2006). In 2012, I moved to Plymouth as a full professor.



Key publications

Wills AJ, Milton F, Longmore CA, Hester S & Robinson J (2013) 'Is overall similarity classification less effortful than single-dimension classification?' Q J Exp Psychol (Hove) 66, (2) 299-318 Author Site , DOI
Wills AJ, Graham S, Koh Z, McLaren IP & Rolland MD (2011) 'Effects of concurrent load on feature- and rule-based generalization in human contingency learning' J Exp Psychol Anim Behav Process 37, (3) 308-316 Author Site , DOI
Milton FN, Longmore, C.A., Wills & A.J. (2008) 'Processes of overall similarity sorting in free classification' Journal of Experimental Psychology: Human Perception and Performance 34, (3) 676-692 Author Site , DOI
Wills AJ, Lavric A, Croft GS & Hodgson TL (2007) 'Predictive learning, prediction errors, and attention: evidence from event-related potentials and eye tracking' J Cogn Neurosci 19, (5) 843-854 Author Site , DOI
Wills AJ, Reimers S, Stewart N, Suret M & McLaren IP (2000) 'Tests of the ratio rule in categorization' Q J Exp Psychol A 53, (4) 983-1011 Author Site , DOI
Wills AJ & McLaren IP (1998) 'Perceptual learning and free classification' Q J Exp Psychol B 51, (3) 235-270 Author Site , DOI
View all publications
Teaching

Teaching

Teaching interests

Research methods, cognitive psychology and neuroscience.

I'm the lead author of the Creative Commons teaching materials Research Methods in R.

Research

Research

Research interests

My principal interest is the categorization of visual objects. This interest has also led me to work on the role of errors and attention in learning more generally. By some metrics, I'm in the top 1% of my field worldwide (the methodology for this calculation can be found here).

I'm also a strong advocate for open science, and the maintainer of a number of R packages, including catlearn (an open repository of learning and categorization models, with over 35,000 downloads to date).

Research degrees awarded to supervised students

2019: Angus Inskter

2016: Charlotte Edmunds

2014: Henrietta Roberts

2014: Fayme Yeates

2006: Fraser Milton

2005: Alice Welham

2004: Kazuhiro Goto

Grants & contracts

2017 - 2020: Learning from total failure (ESRC)

2010 - 2013: The Exeter Science Exchange (EPSRC)

2011 - 2012: confidential project (QinetiQ)

2011 - 2011: IED threat awareness (CDE, Defence)

2009 - 2010: Neuroscience of human associative learning (British Council)

2006 - 2007: Unsupervised categorization (ESRC)

2005 - 2007: From Associations to Rules (EC FP6)

2002 - 2004: Acquisition of polymorphous concepts (BBSRC)

Publications

Publications

Key publications

Wills AJ, Milton F, Longmore CA, Hester S & Robinson J (2013) 'Is overall similarity classification less effortful than single-dimension classification?' Q J Exp Psychol (Hove) 66, (2) 299-318 Author Site , DOI
Wills AJ, Graham S, Koh Z, McLaren IP & Rolland MD (2011) 'Effects of concurrent load on feature- and rule-based generalization in human contingency learning' J Exp Psychol Anim Behav Process 37, (3) 308-316 Author Site , DOI
Milton FN, Longmore, C.A., Wills & A.J. (2008) 'Processes of overall similarity sorting in free classification' Journal of Experimental Psychology: Human Perception and Performance 34, (3) 676-692 Author Site , DOI
Wills AJ, Lavric A, Croft GS & Hodgson TL (2007) 'Predictive learning, prediction errors, and attention: evidence from event-related potentials and eye tracking' J Cogn Neurosci 19, (5) 843-854 Author Site , DOI
Wills AJ, Reimers S, Stewart N, Suret M & McLaren IP (2000) 'Tests of the ratio rule in categorization' Q J Exp Psychol A 53, (4) 983-1011 Author Site , DOI
Wills AJ & McLaren IP (1998) 'Perceptual learning and free classification' Q J Exp Psychol B 51, (3) 235-270 Author Site , DOI

Key publications are highlighted

Journals
Articles
Longman C, Milton F & Wills A (2023) 'Transfer of strategic task components across unique tasks that share some common structures' Quarterly Journal of Experimental Psychology , DOI Open access
Hollins T, Seabrooke T, Inkster A, Wills A & Mitchell C (2022) 'Pre-testing Effects Are Target-Specific and Are Not Driven by a Generalised State of Curiosity' Memory , DOI Open access
Inkster A, Mitchell C, Schlegelmilch R & Wills A (2022) 'Effect of a context shift on the inverse base rate effect' Open Journal of Experimental Psychology and Neuroscience 1, 22-29 , DOI Open access
Spicer S, Mitchell C, Wills A, Blake K & Jones P (2022) 'Theory protection: do humans protect existing associative links?' Journal of Experimental Psychology: Animal Learning and Cognition 48, (1) 1-16 , DOI Open access
Inkster A, Milton F, Edmunds CER, Benattayallah A & Wills A (2021) 'Neural correlates of the inverse base-rate effect' Human Brain Mapping , DOI Open access
Schlegelmilch R, Wills A & von Helversen B (2021) 'A Cognitive Category-Learning Model of Rule Abstraction, Attention Learning, and Contextual Modulation' Psychological Review , DOI Open access
Seabrooke T, Mitchell C, Wills A, Inkster A & Hollins T (2021) 'The Benefits of Impossible Tests: Assessing the Role of Error-Correction in the Pretesting Effect' Memory and Cognition , DOI Open access
Jones PM, Mitchell C, Wills A & Spicer S (2021) 'Similarities and differences: Comment on Chan et al' Journal of Experimental Psychology: Animal Learning and Cognition , DOI Open access
Spicer SG, Wills AJ, Jones PM, Mitchell CJ & Dome L (2021) 'Representing uncertainty in the Rescorla-Wagner model: Blocking, the redundancy effect, and outcome base rate' Open Journal of Experimental Psychology and Neuroscience 1, 14-21 Publisher Site , DOI Open access
Seabrooke T, Mitchell C, Wills A & Hollins T (2020) 'Pretesting boosts recognition, but not cued recall, of targets from unrelated word pairs' Psychonomic Bulletin and Review , DOI Open access
Edmunds C, Inkster A, Jones P, Milton F & Wills A (2020) 'Absence of cross-modality analogical transfer in perceptual categorization' Open Journal of Experimental Psychology and Neuroscience , DOI Open access
Wills A, Ellett L, Milton F, Croft G & Beesley T (2020) 'A dimensional summation account of polymorphous category learning' Learning and Behavior 48, (1) 66-83 , DOI Open access
Spicer S, Mitchell C, Wills A & Jones P (2019) 'Theory protection in associative learning: humans maintain certain beliefs in a manner that violates prediction error' Journal of Experimental Psychology: Animal Learning and Cognition , DOI Open access
Milton F, McLaren IPL, Copestake E, Satherley D & Wills A (2019) 'The effect of pre-exposure on overall similarity categorization' Journal of Experimental Psychology: Animal Learning and Cognition , DOI Open access
Wills A, Edmunds C, Le Pelley M, Milton F, Newell B, Dwyer D & Shanks D (2019) 'Dissociable learning processes, associative theory, and testimonial reviews: A comment on Smith and Church (2018' Psychonomic Bulletin and Review , DOI Open access
Seabrooke T, Mitchell C, Wills A, Waters J & Hollins T (2019) 'Selective effects of errorful generation on recognition memory: The role of motivation and surprise' Memory , DOI Open access
Seabrooke T, Hollins T, Kent C, Wills A & Mitchell C (2018) 'Learning from failure: Errorful generation improves memory for items, not associations' Journal of Memory and Language 104, 70-82 , DOI Open access
Seabrooke T, Wills A, Hogarth L & Mitchell C (2018) 'Automaticity and cognitive control: Effects of cognitive load on cue-controlled reward choice' Quarterly Journal of Experimental Psychology , DOI Open access
Sambrook T, Wills AJ, Hardwick B & Goslin J (2018) 'Model-free and model-based reward prediction errors in EEG' NeuroImage Open access
Lea SEG, Pothos EM, Wills AJ, Leaver LA, Ryan CME & Meier C (2018) 'Multiple Feature Use in Pigeons’ Category Discrimination: The Influence of Stimulus Set Structure and the Salience of Stimulus Differences' Journal of Experimental Psychology: Animal Learning and Cognition 44, (2) 114-127 Open access
Edmunds C, Milton F & Wills AJ (2018) 'Due process in dual process: Model-recovery simulations of decision-bound strategy analysis in category learning' Cognitive Science , DOI Open access
Edmunds CER, Wills AJ & Milton F (2017) 'Initial training with difficult items does not facilitate category learning' Quarterly Journal of Experimental Psychology , DOI Open access
Longman CS, Milton F, Wills AJ & Verbruggen F (2017) 'Transfer of learned category-response associations is modulated by instruction' Acta Psychologica Open access
Wills AJ & Hollins TJ (2017) 'In Defence of Effect-Centric Research' Journal of Applied Research in Memory and Cognition 6, (1) 43-46 , DOI Open access
Wills AJ, O'Connell G, Edmunds CER & Inkster AB (2017) 'Progress in modeling through distributed collaboration: Concepts, tools, and category-learning examples' Psychology of Learning and Motivation , DOI Open access
Lawson R, Chang F & Wills AJ (2016) 'Free classification of large sets of everyday objects is more thematic than taxonomic' Acta Psychologica 26-40 , DOI Open access
Milton F, Bealing P, Carpenter KL, Bennattayallah A & Wills AJ (2016) 'The Neural Correlates of Similarity- and Rule-based Generalization' Journal of Cognitive Neuroscience 29, (1) 150-166 , DOI Open access
Le Pelley ME, Mitchell CJ, Beesley T, George DN & Wills AJ (2016) 'Attention and associative learning in humans: An integrative review' Psychological Bulletin , DOI Open access
O'Connell G, Myers CE, Hopkins RO, McLaren RP, Gluck MA & Wills AJ (2016) 'Amnesic Patients Show Superior Generalization in Category Learning' Neuropsychology , DOI Open access
Carpenter KL, Wills AJ, Benattayallah A & Milton F (2016) 'A Comparison of the neural correlates that underlie rule-based and information-integration category learning' Human Brain Mapping 37, (10) 3557-3574 , DOI Open access
Roberts H, Watkins ER & Wills AJ (2016) 'Does Rumination Cause “Inhibitory” Deficits?' Psychopathology Review a4, (3) 341-376 , DOI Open access
Maes E, De Filippo G, Inkster AB, Lea SEG, De Houwer J, D’Hooge R, Beckers T & Wills AJ (2015) 'Feature- versus rule-based generalization in rats, pigeons and humans' Animal Cognition 18, (6) 1267-1284 , DOI Open access
Wills AJ, Inkster AB & Milton F (2015) 'Combination or Differentiation? Two theories of processing order in classification' Cognitive Psychology 80, 1-33 , DOI Open access
Hogarth L, Zhimin H, Chase HW, Wills AJ, Troisi II J, Leventhal M, Mathew AR & Hitsman B (2015) 'Negative mood reverses devaluation of goal-directed drug-seeking favouring an incentive learning account of drug dependence' Psychopharmacology , DOI Open access
Edmunds CER, Milton F & Wills AJ (2015) 'Feedback can be superior to observational training for both rule-based and information-integration category structures' Quarterly Journal of Experimental Psychology 68, (6) 1203-1222 Author Site , DOI Open access
Yeates F, Wills AJ, Jones FW & McLaren IPL (2014) 'State trace analysis: Dissociable processes in a connectionist network?' Cognitive Science 39, (5) 1047-1061 , DOI Open access
Wills AJ, Lavric A, Hemmings Y & Surrey E (2014) 'Attention, predictive learning, and the inverse base-rate effect: evidence from event-related potentials' Neuroimage 87, 61-71 Author Site , DOI Open access
Roberts H, Watkins ER & Wills AJ (2013) 'Cueing an unresolved personal goal causes persistent ruminative self-focus: an experimental evaluation of control theories of rumination' J Behav Ther Exp Psychiatry 44, (4) 449-455 Author Site , DOI Open access
Yeates F, Jones FW, Wills AJ, McLaren RP & McLaren IPL (2013) 'Modeling human sequence learning under incidental conditions' J Exp Psychol Anim Behav Process 39, (2) 166-173 Author Site , DOI
Newell BR, Moore CP, Wills AJ & Milton F (2013) 'Reinstating the Frontal Lobes? Having More Time to Think Improves Implicit Perceptual Categorization: A Comment on Filoteo, Lauritzen, and Maddox (2010)' PSYCHOLOGICAL SCIENCE 24, (3) 386-389 , DOI
Wills AJ, Milton F, Longmore CA, Hester S & Robinson J (2013) 'Is overall similarity classification less effortful than single-dimension classification?' Q J Exp Psychol (Hove) 66, (2) 299-318 Author Site , DOI
Wills AJ & Pothos EM (2012) 'On the Adequacy of Bayesian Evaluations of Categorization Models: Reply to Vanpaemel and Lee (2012)' PSYCHOLOGICAL BULLETIN 138, (6) 1259-1261 , DOI
Welham AK & Wills AJ (2011) 'Unitization, similarity, and overt attention in categorization and exposure' Mem Cognit 39, (8) 1518-1533 Author Site , DOI
Wills AJ, Graham S, Koh Z, McLaren IP & Rolland MD (2011) 'Effects of concurrent load on feature- and rule-based generalization in human contingency learning' J Exp Psychol Anim Behav Process 37, (3) 308-316 Author Site , DOI
Goto K, Lea SEG, Wills AJ & Milton F (2011) 'Interpreting the effects of image manipulation on picture perception in pigeons (Columba livia) and humans (Homo sapiens)' J Comp Psychol 125, (1) 48-60 Author Site , DOI
McLaren IPL, Wills AJ & Graham S (2011) 'Representation development, perceptual learning, and concept formation' BEHAVIORAL AND BRAIN SCIENCES 34, (3) 141-U41 , DOI
Graham S, Jie HL, Chan H-M, McLaren IPL & Wills AJ (2011) 'Simultaneous Backward Conditioned Inhibition and Mediated Conditioning' JOURNAL OF EXPERIMENTAL PSYCHOLOGY-ANIMAL BEHAVIOR PROCESSES 37, (2) 241-245 , DOI
Hopewell LJ, Leaver LA, Lea SEG & Wills AJ (2010) 'Grey squirrels (Sciurus carolinensis) show a feature-negative effect specific to social learning' Anim Cogn 13, (2) 219-227 Author Site , DOI
Beesley T, Wills AJ & Le Pelley ME (2010) 'Syntactic transfer in artificial grammar learning' PSYCHONOMIC BULLETIN & REVIEW 17, (1) 122-128 , DOI
Wills AJ, Lea SE, Leaver LA, Osthaus B, Ryan CM, Suret MB, Bryant CM, Chapman SJ & Millar L (2009) 'A comparative analysis of the categorization of multidimensional stimuli: I. Unidimensional classification does not necessarily imply analytic processing; evidence from pigeons (Columba livia), squirrels (Sciurus carolinensis), and humans (Homo sapiens)' J Comp Psychol 123, (4) 391-405 Author Site , DOI
Lea SEG, Wills AJ, Leaver LA, Ryan CME, Bryant CML & Millar L (2009) 'A comparative analysis of the categorization of multidimensional stimuli: II. Strategic information search in humans (Homo sapiens) but not in pigeons (Columba livia)' J Comp Psychol 123, (4) 406-420 Author Site , DOI
Milton F, Wills AJ & Hodgson TL (2009) 'The neural basis of overall similarity and single-dimension sorting' Neuroimage 46, (1) 319-326 Author Site , DOI
Milton F & Wills AJ (2009) 'Long-term persistence of sort strategy in free classification' Acta Psychol (Amst) 130, (2) 161-167 Author Site , DOI
Wills AJ (2009) 'Prediction Errors and Attention in the Presence and Absence of Feedback' CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 18, (2) 95-100 , DOI
Milton FN, Longmore, C.A., Wills & A.J. (2008) 'Processes of overall similarity sorting in free classification' Journal of Experimental Psychology: Human Perception and Performance 34, (3) 676-692 Author Site , DOI
Lea SEG & Wills AJ (2008) 'Use of multiple dimensions in learned discriminations' Comparative Cognition & Behavior Reviews 3, , DOI
Haslam C, Wills AJ, Haslam SA, Kay J, Baron R & McNab F (2007) 'Does maintenance of colour categories rely on language? Evidence to the contrary from a case of semantic dementia' Brain Lang 103, (3) 251-263 Author Site , DOI
Wills AJ, Lavric A, Croft GS & Hodgson TL (2007) 'Predictive learning, prediction errors, and attention: evidence from event-related potentials and eye tracking' J Cogn Neurosci 19, (5) 843-854 Author Site , DOI
Lea SEG, Wills AJ & Ryan CME (2006) 'Why are artificial polymorphous concepts so hard for birds to learn?' Q J Exp Psychol (Hove) 59, (2) 251-267 Author Site , DOI
Wills AJ, Noury M, Moberly NJ & Newport M (2006) 'Formation of category representations' Mem Cognit 34, (1) 17-27 Author Site , DOI
Le Pelley ME, Oakeshott SM, Wills AJ & McLaren IPL (2005) 'The outcome specificity of learned predictiveness effects: Parallels between human causal learning and animal conditioning' JOURNAL OF EXPERIMENTAL PSYCHOLOGY-ANIMAL BEHAVIOR PROCESSES 31, (2) 226-236 , DOI
Goto K, Wills AJ & Lea SEG (2004) 'Global-feature classification can be acquired more rapidly than local-feature classification in both humans and pigeons' Anim Cogn 7, (2) 109-113 Author Site , DOI
Milton F & Wills AJ (2004) 'The influence of stimulus properties on category construction' J Exp Psychol Learn Mem Cogn 30, (2) 407-415 Author Site , DOI
Wills AJ, Suret M & McLaren IPL (2004) 'Brief communication: the role of category structure in determining the effects of stimulus preexposure on categorization accuracy' Q J Exp Psychol B 57, (1) 79-88 Author Site , DOI
Wills AJ, Reimers S, Stewart N, Suret M & McLaren IP (2000) 'Tests of the ratio rule in categorization' Q J Exp Psychol A 53, (4) 983-1011 Author Site , DOI
Wills AJ, Reimers S, Stewart N, Suret M & McLaren IPL (2000) 'Tests of the ratio rule in categorization' QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY SECTION A-HUMAN EXPERIMENTAL PSYCHOLOGY 53, (4) 983-1011 , DOI
Wills AJ & McLaren IP (1998) 'Perceptual learning and free classification' Q J Exp Psychol B 51, (3) 235-270 Author Site , DOI
Jones FW, Wills AJ & McLaren IP (1998) 'Perceptual categorization: connectionist modelling and decision rules' Q J Exp Psychol B 51, (1) 33-58 Author Site , DOI
Wills AJ & McLaren IPL (1997) 'Generalization in human category learning: A connectionist account of differences in gradient after discriminative and non-discriminative training' QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY SECTION A-HUMAN EXPERIMENTAL PSYCHOLOGY 50, (3) 607-630 , DOI
Reviews
Wills AJ & Pothos EM (2012) 'On the adequacy of current empirical evaluations of formal models of categorization' Psychol Bull 138, (1) 102-125 Author Site , DOI
Chapters
Lochmann T & Wills AJ (2019) 'Predictive History in an Allergy Prediction Task' Proceedings of EuroCogSci 03 Routledge 217-222 Publisher Site , DOI
Wills AJ (2019) 'Adapting to a Response Deadline in Categorization' Proceedings of the Twenty-Fourth Annual Conference of the Cognitive Science Society Routledge 938-943 Publisher Site , DOI
Zwickel J & Wills AJ (2019) 'Is Competitive Learning an Adequate Account of Free Classification?' Proceedings of the Twenty-Fourth Annual Conference of the Cognitive Science Society Routledge 982-987 Publisher Site , DOI
Kovacs T & Wills AJ (2012) 'Generalization Versus Discrimination in Machine Learning' Encyclopedia of the Sciences of Learning Springer US 1352-1355 , DOI
Pothos EM & Wills AJ (2011) 'Introduction' Formal Approaches in Categorization Cambridge University Press 1-17 , DOI
(2005) 'New Directions in Human Associative Learning' Psychology Press , DOI
Conference Papers
Dome L, Edmunds C & Wills A (2021) 'SUSTAIN captures category learning, recognition, and hippocampal activation in a unidimensional vs information-integration task' Annual Meeting of the Cognitive Science Society 7-/-0/20217-/-0/2021eScholarship Publisher Site Open access
Edmunds CER, Milton F & Wills AJ (2017) 'Due process in dual process: A model-recovery analysis of Smith et al. (2014)' 1979-1984
Edmunds CER, Wills AJ & Milton FN (2016) 'Memory for exemplars in category learning' 2243-2248
Edmunds CER & Wills AJ (2016) 'Modeling category learning using a dual-system approach: A simulation of Shepard, Hovland and Jenkins (1961) by COVIS' 69-74
Inkster AB, Milton FN & Wills AJ (2014) 'Does incidental training increase the prevalence of overall similarity classification? A re-examination of Kemler Nelson (1984)' Austin, TX Cognitive Science Society Open access
Carmantini GS, Cangelosi A & Wills AJ (2014) 'Machine learning of visual object categorization: an application of the SUSTAIN model' Austin, TX Cognitive Science Society 290-295 Open access
Milton F, Copestake E, Satherley D, Stevens T & Wills AJ (2014) 'The effect of pre-exposure on family resemblance categorization for stimuli of varying levels of perceptual difficulty' Austin, TX Cognitive Science Society 1018-1023 Open access
Yeates F, Jones FW, Wills AJ, Aitken MRF & McLaren IPL (2013) 'Implicit Learning: A Demonstration and a Revision of a Novel SRT Paradigm' 3829-3834
Wills AJ, Longmore CA & Milton F (2013) 'Impulsivity and Overall Similarity Classification' 3783-3788
Yeates F, Jones FW, Wills AJ, Aitken MRF & McLaren IPL (2012) 'Implicit Learning: A Demonstration and a Novel SRT Paradigm' 1185-1190
Yeates F, Wills A, Jones F & McLaren I (2012) 'State-Trace Analysis of Sequence Learning by Recurrent Networks' 2581-2586
Milton F, Viika L, Henderson H & Wills AJ (2011) 'The effect of time pressure and the spatial integration of the stimulus dimensions on overall similarity categorization' 795-800
Wills AJ, Barrasin TJ & McLaren IPL (2011) 'Working Memory Capacity and Generalization in Predictive Learning' 3205-3210
Lavric A & Wills A (2005) 'ERPS support attentional theories of associative learning' 171-171
Wills AJ, Ellett L & Lea SEG (2004) 'Acquisition of polymorphous concepts' 1657-1657
Welham A & Wills AJ (2004) 'Emergence of features in visual stimuli' 1654-1654
Welham AK, Schnadt MJ & Wills A (2003) 'Speeded categorization: The effects of perceptual processing and decision-making time' 1218-1223
Wills AJ, Suret M & McLaren IPL (2000) 'Categorization and the Ratio Rule' 994-999
Wills AJ & McLaren IPL (1997) 'Generalization in category learning: One and two category problems' 1088-1088
Presentations and posters
Schlegelmilch R, Wills AJ & von Helversen B Schlegelmilch R, Wills AJ & von Helversen B 'CALM - A Process Model of Category Generalization, Abstraction and Structuring'