Staff Profile

I am a biologist and psychologist whose primary research interest is social evolution, and more recently the application of behavioural science to social policy. I am also interested in ethics and work in public engagement, details of which follow below.

Google Scholar: Click here.

Public engagement

Winner of public engagement award from the North East of England Branch of the British Psychological Society, February 2017.

Public engagement officer for the Centre for Behaviour and Evolution.

Consultant and contributor on science and arts/science collaborations for the printed media, radio, theatre, cinema, academia and The Poetry Society.

2018: Fulcrum Arts and Research. Contribution to examples of practice in public engagement, Association for Academic Outreach, Lancaster University (Example of practice) and to submission of evidence by the Association to the Civic University Commission (Submission of evidence).

2017: Advisor to Bridge+Tunnel Productions and film maker Dr Tina Gharavi on her documentary Tribalism is Killing Us on the themes of discrimination and racism (https://vimeo.com/204184305). 

2017: Invited contribution, 'Cooperation and adversity: evolutionary and cultural perspectives', to a conference on the project in the following item here. January. See also Lazarus (2017 Global Discourse) under Publications tab.

2015: Led the Fulcrum event on social dilemmas for Dr Matthew Johnson’s cross-cultural project ‘Good Culture and Precariousness’, Ashington, Northumberland. June. For my report on the event, 'Cooperation Games Event, Ashington', see here.

2015: Two workshops on ‘Correlation and Causation’ in the series of ‘Science Tips for Policy Makers’, Fulcrum Arts and Research workshops based on Nature Comment paper. Culture Lab, Newcastle University, March-April.

2014-2016: Psychological consultant to Improbable theatre company on ‘Opening Skinner's Box’, devised theatre show at Northern Stage, Newcastle (April 2016) and other venues in the UK and US, based on 20^th century psychology studies. September 2014-April 2016.

2014: Filmed interview on the evolution of human cooperation for the project ‘Good Culture and Precariousness’ by Dr Matthew Johnson (University of Lancaster). Interview available, August.

2014: Presented Café Scientifique, ‘Dancing Donkeys Pub Quiz’, a Fulcrum Arts and Research event on social dilemma games, Empty Shop HQ, Durham. June.

2014: Discussant on the evolutionary biology of community, Discussion Evening at the exhibition ‘Transformation Content’ by Louise Mackenzie, The Holy Biscuit Gallery, Newcastle. March.

2013: Presented ‘The Big Society – The role of Evolutionary Biology in Understanding Cooperation’, a Fulcrum Arts and Research workshop in which a scientific talk is followed by the audience explaining the science through artistic means (writing, performance and visual art) Newcastle University, November.

2013: Interview with public audience, and demonstration of cooperation game, for British Science Festival’s Xchange team on ‘The Big Society? Interactive Psychology Games’ event (podcast). September.

2013: Organizer of the British Science Festival event: ‘The Big Society? Interactive Psychology Games’ . September.

2013: ‘Game of Life Surprises’: article in The Journal, (Newcastle daily paper) pp. 19-21, on forthcoming British Science Festival event, ‘The Big Society? Interactive Psychology Games’. 18 July.

2013: Discussant, with Dr John Gowing, of the film Darwin's Nightmare, Cinema Politica, Newcastle University. June.

2013: ‘Dancing Donkeys Pub Quiz’, a Fulcrum Group event on social dilemma games, ‘Convention, Habit or Custom’ project, Newcastle. May.

2013-2018: Director of Research, Fulcrum Arts and Research (‘Understanding Science through the Arts’).

2013: Film presentation and Q&A: Crimes and Misdemeanors (dir. Woody Allen), first in the SciScreen series, Tyneside Cinema, Newcastle. February.

2013: Contribution on SciScreen Newcastle to ‘British Science Festival 2013: Get Ready!’ meeting, Great North Museum: Hancock, Newcastle. January.

2012-2013: Answers to science questions from the public in advance of the British Science Festival.

2012: Social dilemma games. Contribution to The Festival of the Apocalypse, an event in The Encampment of Eternal Hope exhibition, Baltic Gallery, Gateshead. December.

2012: Lessons to two Year 4 classes at Grafton Primary School, Holloway, London, on Darwin and adaptation. October.

2012: Lecture on ‘Darwin and Evolution’ to Northumberland Wildlife Trust, Coquetdale Branch, Rothbury. March.

2012: Lecture on ‘Evolution, Extinction and the Lost Humans’ with Daniel Nettle, for National Science & Engineering Week, Great North Museum: Hancock, Newcastle. March.

2012: Lecture on ‘Darwin and Evolution’ to Alnwick U3A. September.

2011-ongoing: Public engagement officer, Centre for Behaviour and Evolution, Newcastle University.

2010: Member of think-tank: The Rules of the Playground (A Theory of Difference): Outsiders, Identity & How Power is Negotiated. 

2010: Lecture on ‘Darwin and Evolution’ to Coquetdale U3A, Rothbury.

2010: Co-director with Lady Kitt of the exhibition Communic8 2 Co-oper8: The Cautious Optimist, an arts/science collaboration, Gallery of Wonder, Newcastle University, 5th October - 5th November. 

2009-10: Reading group leader for Darwin’s ‘Origin of Species’ and ‘The Descent of Man’, Literary and Philosophical Society, Newcastle and Newcastle University.

2009: Lecture on ‘Darwin and Evolution’ to the Rothbury and Coquetdale History Society and the Coquetdale Group of Northumberland Wildlife Trust, Rothbury.

2009: Textual contributions to Darwin Exhibition, University Library, Newcastle University.

2009: Teaching natural selection to KS4 and KS5 students, ‘Evolve!’ workshop, Great North Museum: Hancock, Newcastle.

2009: Introduction and Q&A for Darwin Festival films Creation and Inherit the Wind. Tyneside Cinema, Newcastle. October-November.

2009: Darwin Quiz, Students’ Union, Newcastle University.

2009: Interview on Darwin for MA radio project.

2009: Member of Darwin Bicentenary Festival committee, Newcastle University.

2007 & 2008: Workshops on animal communication for ‘Setting Language Research to Music’ projects for secondary school students, School of English, Newcastle University, leading to performances at The Sage, Gateshead and CD recordings.

2005-2006: Consulted by BBC News Online and other media on science stories.

2005: Poetry and Science Experiment, Newcastle, organised by The Poetry Society, with presentation on National Poetry Day, Literary and Philosophical Society, Newcastle. 6 October.

2005: Contribution to BBC Radio 4 and World Service science programme on sex ratios.

2004: Science advisor to Live Theatre, Newcastle 'Distillations' youth science/drama project, funded by Wellcome Trust.

2002-05: UK and overseas magazine interviews on the subject of human sex ratios.

1991: Invitation from Radio 4 concerning anti-predator research in Serengeti gazelles.

1990: Interview for local newspaper on anti-predator research in Serengeti gazelles.

1987: Interview for national newspaper on the biology and evolution of sexual behaviour.

1973: Field project on the behaviour of the pink-footed goose: environmental impact of Icelandic hydro-electric scheme.

Area of expertise: evolutionary analysis of behaviour

Research Interests

Negativity bias

I have recently proposed a hypothesis for the evolution of the psychological phenomenon of negativity bias, together with an empirical programme for its further analysis.

Cooperation: Evolution and Social Policy

I am working to understand how cooperation has evolved in the organic world, and particularly in the human species. I am currently analysing how cooperation varies with adversity and uncertainty, and how it coevolves with intention signals.

The solution to many problems in social policy – climate change negotiation, environmental degradation, vaccination hesitancy and many others – hinges on finding ways to encourage players to cooperate for the common good. In 2023 the journal Global Discourse published my edited issue ‘Cooperation and social policy: Integrating evidence into practice’ that brings together behavioural researchers with expertise in cooperation and social policy practitioners to examine how the research on cooperation can be brought to bear on these problems.

Trade-Offs in Human Mate Choice

Studies of human mate choice tend to focus on a single attribute of choice, whereas choosing a mate involves simultaneous judgement of a number of attributes. I have developed a method for measuring the way in which people trade off the importance of two different attributes in judging the desirability of a potential partner. The method allows quantification of the equivalence of two different mate qualities; for example, the degree of facial attractiveness a woman would forgo in one partner in order to gain another who had, say, an additional £20,000 a year in salary. We have used the method to test hypotheses about the relative importance of different mate qualities as a function of variables of adaptive interest, such as age, relationship status and the nature of the relationship sought.

Esteem Indicators

Journal Editorships

1996-1998: Animal Behaviour.

2001-2004: Ethology.

2011-: Advisory Editorial Board Member, Global Discourse: An Interdisciplinary Journal of Current Affairs and Applied Contemporary Thought.

Appointments on Scientific Bodies

1981, 1984: Consulting Editor, Animal Behaviour.

1981-84, 1991-93: Council member, Association for the Study of Animal Behaviour.

1983-1992: Council member, International Council of Ethologists.

1985-90: Hon. Treasurer, Company Secretary, Chairman of Grants Committee, and Secretary of the Finance and Journal Committees of the Association for the Study of Animal Behaviour.

Other Work for External Bodies

Research strategy adviser, Psychology, University of Edinburgh.

Member of experts panel and media panel, British Psychological Society.

PSY1006: Instinct, Learning and Motivation. Contributing lectures and assignment. Up to and including 2020-21.

PSY3001: Evolution and Behaviour. Module leader from 2016-17 to 2018-19.

• Articles

  • Lazarus J. Designing a social policy initiative for greater cooperation: social dilemmas, behavioural game theory, motives, altruism and ethics. Global Discourse 2023, 13(3-4), 452-468.
  • Lazarus J. Cooperation and Social Policy: Integrating Evidence into Practice – introduction to the issue. Global Discourse 2023, 13(2), 119-135.
  • Lazarus J. Negativity bias: An evolutionary hypothesis and an empirical programme. Learning and Motivation 2021, 75, 101731.
  • Lazarus J. Moral relevance of cognitive complexity, empathy and species differences in suffering. Animal Sentience 2019, 3(23), 43.
  • Lazarus J. Cooperation in adversity: An evolutionary approach. Global Discourse 2017, 7(4), 571-598.
  • Andras P, Lazarus J, Roberts G. Environmental adversity and uncertainty favour cooperation. BMC Evolutionary Biology 2007, 7(1), 240.
  • Farrelly D, Lazarus J, Roberts G. Altruists Attract. Evolutionary Psychology 2007, 5(2), 313-329.
  • Andras P, Lazarus J, Roberts G, Lynden SJ. Uncertainty and Cooperation: Analytical Results and a Simulated Agent Society. Journal of Artificial Societies and Social Simulation 2006, 9(1).
  • Lazarus J. Sociosexuality and sex ratio: Sex differences and local markets. Behavioral and Brain Sciences 2005, 28(2), 288.
  • Lazarus J, Inglis IR, Torrance RLLF. Mate Guarding Conflict, Extra-Pair Courtship And Signalling in the Harlequin Duck, Histrionicus Histrionicus. Behaviour 2004, 141(8), 1061-1078.
  • Lazarus J. Let's cooperate to understand cooperation. Behavioral and Brain Sciences 2003, 26(2), 169-170.
  • Scannell J, Roberts G, Lazarus J. Prey scan at random to evade observant predators. Proceedings of the Royal Society B: Biological Sciences 2001, 268(1466), 541-547.
  • Inglis IR, Langton S, Forkman B, Lazarus J. An information primacy model of exploratory and foraging behaviour. Animal Behaviour 2001, 62(3), 543-557.
  • Inglis IR, Lazarus J, Torrance RLLF. Breeding status and aggressive communication in the Harlequin Duck Histrionicus histrionicus. Wildfowl 2000, 51, 139-153.
  • R M Neems,J Lazarus,A J Mclachlan. Lifetime reproductive success in a swarming midge: trade-offs and stabilizing selection for male body size. Behavioral Ecology 1998, 9, 279-286.
  • Inglis IR, Forkman B, Lazarus J. Free food or earned food? A review and fuzzy model of contrafreeloading. Animal Behaviour 1997, 53(6), 1171-1191.
  • Huntingford FA, Lazarus J, Barrie BD, Webb S. A dynamic analysis of cooperative predator inspection in sticklebacks. Animal Behaviour 1994, 47(2), 413-423.
  • Neems RM, Lazarus J, Mclachlan AJ. Swarming behaviour in male chironomid midges: a cost-benefit analysis. Behavioral Ecology 1992, 3(4), 285-290.
  • Lazarus J, Symonds M. Contrasting effects of protective and obstructive cover on avian vigilance. Animal Behaviour 1992, 43(3), 519-521.
  • K Warburton,J Lazarus. Tendency-distance models of social cohesion in animal groups. Journal of Theoretical Biology 1991, 150, 473-488.
  • Lazarus J, Metcalfe NB. Tit-for-tat cooperation in sticklebacks: a critique of Milinski. Animal Behaviour 1990, 39(5), 987-988.
  • Lazarus J. The logic of mate desertion. Animal Behaviour 1990, 39(4), 672-684.
  • J Lazarus. Looking for trouble. (Feature article). New Scientist 1990, 125(1701), 62-65.
  • I R Inglis,J Lazarus,R Torrance. The pre-nesting behaviour and time budget of the harlequin duck Histrionicus histrionicus. Wildfowl 1989, 40, 55-73.
  • Lazarus J, Inglis IR. Shared and unshared parental investment, parent-offspring conflict and brood size. Animal Behaviour 1986, 34(6), 1791-1804.
  • I R Inglis,J Lazarus. Vigilance and flock size in brent geese: the edge effect. Zeitschrift fur Tierpsychologie 1981, 57, 193-200.
  • Lazarus J. The early warning function of flocking in birds: an experimental study with captive Quelea. Animal Behaviour 1979, 27(3), 855-865.
  • J Lazarus. Flock size and behaviour in captive red-billed weaverbirds (Quelea quelea): implications for social facilitation and the functions of flocking. Behaviour 1979, 71, 127-145.
  • J Lazarus. Vigilance, flock size and domain of danger size in the white-fronted goose. Wildfowl 1978, 29, 135-145.
  • J Lazarus,I R Inglis. Breeding behaviour of the pink-footed goose: parental care and vigilant behaviour during the fledging period. Behaviour 1978, 65, 62-88.
  • I R Inglis,J Lazarus,B N Shephard. The behaviour of the pink-footed goose on its Icelandic breeding grounds. Wildfowl 1976, 27, 159.
  • S Dimond,J Lazarus. The problem of vigilance in animal life. Brain, Behaviour and Evolution 1974, 9, 60-79.
  • Lazarus J, Crook JH. The effects of luteinizing hormone, oestrogen and ovariectomy on the agonistic behaviour of female Quelea quelea. Animal Behaviour 1973, 21(1), 49-60.
  • J Lazarus. Natural selection and the functions of flocking in birds: a reply to Murton. Ibis 1972, 114, 556-558.

• Book Chapters

  • Lazarus, J. Cooperation in adversity: an evolutionary approach. In: Matthew Johnson, ed. Precariousness, Community and Participation. Abingdon: Routledge, 2018, pp.125-152.
  • Andras P, Lazarus J. Cooperation, Risk and the Evolution of Teamwork. In: Gold, N, ed. Teamwork: Multi-Disciplinary Perspectives. Basingstoke: Palgrave Macmillan, 2005, pp.56-77.
  • Lazarus J. Altruism and cooperation. In: A. Kuper J. Kuper, ed. The Social Science Encyclopedia. London and New York: Routledge, 2004, pp.22-25.
  • Andras P, Roberts G, Lazarus J. Environmental risk, cooperation, and communication complexity. In: Alonso E; Kudenko D; Kazakov D, ed. Adaptive agents and multi-agent systems : adaptation and multi-agent learning. Berlin: Springer, 2003, pp.558-560.
  • Lazarus J. Human sex ratios: adaptations and mechanisms, problems and prospects. In: Hardy ICW, ed. Sex Ratios: Concepts and Research Methods. Cambridge: Cambridge University Press, 2002, pp.287-311.
  • J Lazarus. Altruism. In: A Kuper;J Kuper, ed. The Social Science Encyclopedia. London and New York: Routledge, 1996, pp.19-20.
  • J Lazarus. Behavioural ecology and evolution. In: D Kimble;A M Colman, ed. Biological aspects of behaviour. London: Longman, 1995, pp.28-46.
  • C D Fitzgibbon,J Lazarus. Anti-predator behaviour of Serengeti ungulates: individual differences and population consequences. In: A R E Sinclair;P Arcese, ed. Serengeti II: Dynamics, Management and Conservation of an Ecosystem. Chicago: University of Chicago Press, 1995, pp.274-296.
  • Hilborn R, Georgiadis N, Lazarus J, et al. A model to evaluate alternative management policies for the Serengeti-Mara ecosystem. In: Sinclair, ARE & P. Arcese, P, ed. Serengeti II: Dynamics, Management and Conservation of an Ecosystem. Chicago: University of Chicago Press, 1995, pp.617-637.
  • J Lazarus. Behavioural ecology and evolution. In: A M Colman, ed. Companion Encyclopedia of Psychology. London: Routledge, 1994, pp.69-87.
  • J Lazarus. Exemplar study contribution to manual for software for behavioural research. In: Noldus Information Technology b.v, ed. The Observer. Wageningen: Noldus Information Technology b.v, 1993.
  • J Lazarus. The concepts of sociobiology. In: A M Colman H Beloff, ed. Psychology Survey 6. Leicester & Cambridge: British Psychological Society & MIT Press, 1987, pp.192-217.
  • J Lazarus. Competition and conflict in animals. In: A M Colman, ed. Cooperation and Competition in Humans and Animals. Wokingham: Van Nostrand Reinhold, 1982, pp.26-56.
  • J Lazarus. Vigilance. In: D J McFarland, ed. The Oxford Companion to Animal Behaviour. Oxford: Oxford University Press, 1981, pp.579-581.

• Conference Proceedings (inc. Abstracts)

  • Andras P, Roberts G, Lazarus J. Environmental risk, cooperation and communication complexity. In: AISB '02 Symposium on Adaptive Agents and Multi-agent Systems. 2002, Imperial College, London: London: Society for the Study of Artificial Intelligence and Simulation of Behaviour.
  • Lazarus J. Sociobiology and human-behavior: principles, predictions and evidence. In: British Psychological Society Annual Conference - Social Psychology Section. 1981, University of Kent, Canterbury: British Psychological Society.

• Edited Books

  • Lazarus J, ed. Cooperation and social policy: Integrating evidence into practice (Global Discourse journal issue). Bristol: Bristol University Press, 2023.
  • Lazarus J, ed. Assessments and Decisions. A Study of Information Gathering by Hermit Crabs, by Elwood R. W. & Neil S. London: Chapman & Hal, 1992.
  • Lazarus J, ed. Social Behaviour in Fluctuating Populations, by Cockburn, A. London & New York: Croom Helm, 1988.
  • Lazarus J, ed. Primate Social Systems, by Dunbar R.I.M. 373pp. London: Croom Helm & Cornell University Press, 1988.
  • Lazarus J, ed. Modelling in Behavioural Ecology, by Lendrem, D. London: Croom Helm, 1986.
  • Lazarus J, ed. Gulls and Plovers: the Ecology and Behaviour of Mixed-Species Feeding Groups, by Barnard C.J. & Thompson D.B.A. London: Croom Helm, 1985.

• Editorials

  • Lazarus J. Cooperation and Social Policy: Integrating Evidence into Practice – introduction to Part 2. Global Discourse 2023, 13(3-4), 240-242.
  • Lazarus J. Cooperation and Social Policy: Integrating Evidence into Practice – introduction to Part 1. Global Discourse 2023, 13(2), 136-137.

• Generic

  • J Lazarus. Wildlife research in Serengeti. The Newsletter of the University of Newcastle upon Tyne 1992. (39), 3.
  • J Lazarus. Birds' variable vigilance. The Newsletter of the University of Newcastle upon Tyne 1992. (38), 8.
  • J Lazarus. Science, evolution and creationism. Education Otherwise Newsletter 1988. (63), 8-9.
  • J Lazarus,I R Inglis,R Torrance. The breeding of the harlequin duck in Iceland. WAGBI Magazine 1979. (Autumn), 12-15.
  • J Lazarus,I R Inglis. Thjorsarver: the threatened breeding ground of the pink-footed goose. WAGBI Magazine 1976. (Summer).
  • J Lazarus,I R Inglis. The pink-footed goose in Iceland. WAGBI Magazine 1975. (Spring), 15-17.
  • I R Inglis,J Lazarus,B Shephard. The behaviour of the pink-footed goose. WAGBI Magazine 1974. (Spring), 22-25.

• Letters

  • Lazarus J. Did the Scottish National Party benefit from ingroup bias?. The Psychologist 2015, 28(8), 610.
  • Lazarus J. Reflexivity and the evolving debate. Psychologist 2001, 14(11), 572-573.
  • Lazarus J. Ethical issues in psychobiological research on animals. Bulletin of the British Psychological Society 1987, 40, 99.
  • Lazarus J. Ethical issues in psychobiological research on animals. Bulletin of the British Psychological Society 1987, 40, 262.

• Notes

  • Lazarus J, Dunbar R. John Hurrell Crook BSc, PhD, DSc (Obituary). ASAB Newsletter 2011, Summer, 2-3.
  • Lazarus J, Dunbar R. John Hurrell Crook BSc, PhD, DSc (Obituary). ISBE Newsletter 2011, 23(2), 1-2.
  • Lazarus J, Dunbar R. John Crook (Obituary). The Times (London) 2011, 6 October, 70.
  • Lazarus J. Vigilance and group size: Early studies, the edge effect, secondary defences, the double advantage trade-off and the future. Behavioural Processes 2003, 63(3), 129-131.

• Online Publication

  • Lazarus, J. Cooperation: From social dilemma to social policy. Bristol, UK: Bristol University Press, Policy Press, 2024. Available at: https://www.transformingsociety.co.uk/2024/05/30/cooperation-from-social-dilemma-to-social-policy/.

• Reports

  • Lazarus J. Predator monitoring in Thomson's gazelle. Serengeti Wildlife Research Centre, 1993. Scientific Report 1990-1992.
  • Lazarus J, Inglis IR, Torrance R. Aggressive displays of the harlequin duck. Preliminary report. National Research Council of Iceland, 1981.
  • Lazarus J, Inglis IR, Torrance R. Breeding behaviour of the harlequin on the River Laxa, Myvatn. Preliminary report. National Research Council of Iceland, 1978.
  • Lazarus J, Inglis IR. The breeding behaviour of the pink-footed goose in Thjorsarver during the fledging period. Orkustofnun, Reykjavik, Iceland, 1977.
  • Lazarus J. Analysis of goose behaviour using SPSS. Contribution to report. Computing Laboratory, University of Newcastle upon Tyne, 1976.
  • Lazarus J, Inglis IR. Ethology of the pink-footed goose at Selmyrar, Iceland. Museum of Natural History, Reykjavik, Iceland, 1973.
  • Lazarus J, Inglis IR. Ethology of the pink-footed goose (Anser brachyrhynchus) at Thjorsarver, Iceland 1973. Preliminary report No. 2: behaviour on the feeding areas. Museum of Natural History, Reykjavik, Iceland, 1973.
  • Lazarus J. A classification of spacing variables and some sources for spatial analysis. Paper circulated at the Working Group Meeting on the Analysis of Spatial Aspects of Behaviour. University of Bristol, 1973.

• Reviews

  • Lazarus J. The Narrow Road Taken. Review of Nature’s Oracle: The Life and Work of W. D. Hamilton by Ullica Segerstrale. Oxford: Oxford University Press. 2013. Journal of Evolutionary Psychology 2014, 12(2-4), 149-152.
  • Lazarus J. Review of the new journal Adaptive Behaviour, edited by J-A Meyer. MIT Press. Times Higher Education Supplement 1994, No. 1125, 28-29.
  • Lazarus J. Review of 'The Observer', version 3.0, software for behavioural research, by Noldus Information Technology b .v., 1993. Psychology Software News 1994, 4, 50-51.
  • Lazarus J. Contribution to multiple review of: 'The Observer' version 2.0, software for behavioural research, by Noldus Information Technology b.v., 1989. Ethology, Ecology and Evolution 1992, 4, 403.
  • Lazarus J. Review of The One Percent Advantage: The Sociobiology of Being Human by J. & M. Gribbin. Oxford: Basil Blackwell, 1988. Journal of Biosocial Science 1990, 22, 265-267.
  • Lazarus J. Review of Analyses in Behavioural Ecology: A Manual for Lab and Field by L. Brown & J.F. Downhower. Sunderland, Mass.: Sinauer, 1988. Biological Psychology 1989, 29, 102-104.
  • Lazarus J. Review of Current Problems in Sociobiology edited by King's College Sociobiology Group, Cambridge. Cambridge: Cambridge University Press. 1982. Human Ethology Newsletter 1984, 4 (2), 4-5.
  • Lazarus J. Review of Genetic Models of Sexual Selection by P. O'Donald. Cambridge: Cambridge University Press. 1980. Behavioural Processes 1981, 6, 386-388.