This article gives a brief overview of activity in the field of parapsychology across continental Europe (mainly experimental). See also Experimental Parapsychology in the UK, Psi Research in France and Psi Research in Russia.
Austria
Austria became a centre for mediumship research after World War I, amid a wave of public interest in spiritualism and survival of death. Key subjects included Willy and Rudi Schneider and Maria Silbert. Investigation of Eleonore Zugun, a Romanian girl who produced spontaneous psychokinetic phenomena, led to the founding in 1927 of the Austrian Society for Psychical Research by Countess Zoë Wassilko and Professor Hans Thirring, a theoretical physicist at the University of Vienna.
In the 1930s, experiments were carried out in long distance telepathy, with subjects situated in major European cities.
Professor Gustav Adolf Schwaiger, technical director of the Austrian Broadcast Corporation, carried out experiments with Rudi Schneider until his laboratory was destroyed by bombing during World War II. His files are preserved at the Society for Psychical Research’s archive in Cambridge.
In 1971, the organization was renamed Austrian Society for Parapsychology, and 1997, ‘Border Areas of Sciences’ was added to its name to highlight its interdisciplinary network with associated fields, following the international trend towards a single integrated field of ‘anomalistics’. It was led for three decades up to 1997 by Professor Helmut Hofmann, an electrical engineering theorist at the Technical University of Vienna. During his term an electronic device for ESP testing was constructed by one of his students. He was succeeded by Professor Manfred Kremser, an anthropologist at the University of Vienna, until 2013. Another key figure in the organization is Professor Peter Mulacz, executive vice president.
The Society serves as an organizational umbrella for research projects by individual members funded by other agencies. These include a re-evaluation of the Zugun case by Mulacz, and, more recently, research by A Batthyány on retroactive/precognitive priming and retrocausal habituation.1
Germany
Institut für Grenzgebiete der Psychologie und Psychohygiene (IGPP)
The Institut für Grenzgebiete der Psychologie und Psychohygiene (IGPP; Institute for Frontier Areas of Psychology and Mental Health) was founded in 1950 by psychologist and physician Hans Bender (1907–1991), the leading figure in post-World War II parapsychology. Bender’s main research activities were ‘poltergeist’ activity and the problem of precognition.
Current researchers include Wolfgang Ambach,2 Harald Atmanspacher,3 Eberhard Bauer, Wolfgang Fach, Andreas Fischer, Liane Hoffmann, Jürgen Kornmeier, Gerhard Mayer, Uwe Schellinger, Michael Schetsche, Ina Schmid Knittel, Dieter Vaitl and Marc Wittmann.
The institute engages in systematic, interdisciplinary research concerning anomalies at the frontiers of current scientific knowledge – altered states of consciousness, exceptional human experiences, mind-matter relations – and their social, cultural and historical contexts, from the perspectives of the humanities, social sciences and natural sciences. It offers information, education and counseling for people with exceptional experiences, also a comprehensive special library and a research archive for parapsychology and frontier areas of psychology.
The Institute is ideologically neutral and institutionally independent. It cooperates with numerous domestic and international universities and research institutions, and contributes to the education of undergraduate and graduate students.
Main research areas of the IGPP are:
(1) Natural sciences and experimental research: generalized quantum theory, mind-matter interactions,4 physiology of subjective evidence (deception, lies, pseudo-memories), time and space as fundamental dimensions of consciousness, altered states of consciousness.
(2) Social and cultural scientific research: prevalence and patterns of extraordinary (anomalous) experiences; 5 social relationships between stigmatization, anomalies and heterodoxies in the construction of reality, history of the reception of parapsychology and anomalistic research, relationships between spirituality religion and psychotherapy.
(3) Counseling, information and educational program for people reporting exceptional experiences; conceptual development and empirical testing of specific diagnostic tools and treatment approaches for people with such experiences; educational workshops for therapists and counsellors.
Gesellschaft für Anomalistik, Freiburg
The Gesellschaft für Anomalistik eV, (Society for Anomalistics), located in Freiburg, seeks to promote and cultivate an informed dialogue between proponents and opponents of controversial scientific claims and seemingly incompatible epistemic positions. Since 2001, it has published Zeitschrift für Anomalistik, edited by Gerhard Mayer, a journal devoted to empirical research reports, general treatises on methodological, conceptual, philosophical or scientific historical aspects, review articles, commentaries and discussion papers, and book reviews about scientific anomalies and exceptional human experiences.
Wissenschaftlichen Gesellschaft zur Förderung der Parapsychologie (WGFP)
Also in Freiburg, the Wissenschaftlichen Gesellschaft zur Förderung der Parapsychologie (Scientific Society for the Promotion of Parapsychology, WGFP), founded 1989, carries out experimental work and counseling in parapsychology, led by Walter von Lucadou.
Since 1990, the WGFP has published Zeitschrift fir Parapsychologie und Grenzgebiete der Psychologie, edited by von Lucadou and Eberhard Bauer, the only German language journal exclusively dedicated to scientific parapsychological research. It mainly contains reports of new experimental and theoretical research, and reviews and case studies concerning the historical development, methods, results, problems and theories of parapsychology.
Suitbert Ertel
A prominent German researcher was Suitbert Ertel (1932–2017), emeritus of Göttingen University. His first interest was to look for empirical evidence of statistical correlations between solar activity and mental mass processes (revolutions) on Earth – a claim made by Alexander Chizhevsky. He found that historical changes of creativity, covarying in independent world cultures, could be added as discoveries of solar-dependent human events.6
He also adopted new methods to partially confirm Michel Gauquelin’s claim of covariance between the number of births of exceptionally creative and renowned individuals with the daily movements of Mars, Jupiter, and Saturn.
More recent experimental contributions were related to the Ball-Selection-Test (BST), a new strategy for screening psi-gifted participants that has proved superior to selection by questionnaire. BST yields not only primary psi effects (psi hitting), but also secondary psi indicators, and enables testing of phenomena such as psi missing, target displacement, hit spreading and bunching, decline effects, inter-individual real time covariation, and subconscious arithmetic operations.7
Other Researchers
Markus Maier, a professor at the University of Munich’s Emotion and Motivation laboratory, has developed a robust program investigating psi phenomena, including replicating Daryl Bem’s implicit precognition paradigm, in particular, the retroactive avoidance of future negative stimuli. Over seven experiments, he and his colleagues found an average effect size of 0.07 – hugely significant statistically.8
Harald Walach (Viadrina University Frankfurt /Oder), is contributing to the Generalized Quantum Theory that could encompass many parapsychological phenomena.9
Stefan Schmidt, at the University of Freiburg, contributed to an appraisal of the evidence related to Distant Mental Interaction with Living Systems (DMILS).10
France
Institut Métapsychique International
In 1919, the Institut Métapsychique International (IMI), was founded in Paris to carry out psychic research, or métapsychique as it is termed in France. Unlike its British and American counterparts the organization was equipped from the first with a laboratory for controlled experiments.
Mario Varvoglis and his collaborators are carrying out investigations related to ESP in altered states of consciousness and exceptional experiences, using innovative methods.11 Recently, they have been joined by physicist Peter Bancel, reanalyzing data on mind-matter interactions, from the lab and from the Global Consciousness Project, a seventeen-year exploration of the effect of global events of mass attention on the behavior of a network of random number generators.
CIRCEE
The Center for Information, Research, and Counseling on Exceptional Experiences (Centre d’Information, de Recherche et de Consultation Sur les Expériences Exceptionelles) is pursuing research on how and why people experience anomalous experiences, led by Thomas Rabeyron, David Acunzo and Renaud Evrard. This information has practical uses in a clinical context, through a counseling service for those who encounter difficulties related to these experiences.
See also Psi Research in France.
Italy
Società Italiana di Metapsichica
The first national organization devoted to psychic studies in Italy was the Società Italiana di Metapsichica (SIM), established in Rome in 1937. Its founders were Ferdinando Cazzamalli, a medical doctor and professor of Neuropsichiatry at Universita' di Roma; Giovanni Schepis, professor of statistics at Università di Roma; Emilio Servadio, psychology professor; and Luigi Sanguineti, neuropsychiatrist. Its approach was similar the British Society for Psychical Research (SPR), to promote rigorous scientific examination of so-called psychic and paranormal phenomena, including state-of-art psycho-bio-physical approaches in experimental work, excluding philosophical or religious associations.
Centro Studi Parapsicologici
In 1954, the Centro Studi Parapsicologici (CSP) was founded in Bologna, headed by Cassoli Piero and Enrico Marini, to study individuals with psychic capacities.
EvanLab and Patrizio Tressoldi
Established in Florence in 2012, the EvanLab is carrying out projects in mental telecommunication and mind-matter interaction at a distance. It is collaborating with Patrizio Tressoldi, who is affiliated with the Dipartimento di Psicologia Generale of Padova University, partially funded by the BIAL Foundation.
Tressoldi is focusing on summarizing available evidence through meta-analyses12 and devising practical applications for some of the phenomena related to these lines of research.13 One is a highly significant pilot EEG entanglement study investigating telepathic responses to stimuli in bonded couples by means of EEG correspondence.14
Netherlands
Experimental work in the Netherlands is being led by Dick Bierman and Jacob Jolij at the Heymans Group for Anomalous Cognition, Department of Experimental Psychology, University of Groningen. The group is continuing the long-running work on presentiment begun by Bierman in 1997 at the University of Amsterdam. The current experimental paradigm was developed in the 1990s by Dean Radin, then at the university of Las Vegas. Experimental work in this area has recently been summarized in a meta-analysis and a theoretical paper by Mossbridge, Tressoldi and Utts15 and Mossbridge, Tressoldi, Utts, Ives, Radin & Jonas.16. See Presentiment.
Belief, Perception and Cognition Lab (Jolij Lab), Groningen
The Belief, Perception and Cognition lab in Groningen, directed by assistant professor Jacob Jolij, is studying the role of belief in perception, using methods of cognitive neuroscience and experimental psychology. It follows two main lines of psi-related research: the effects of random number generator source on perceptual accuracy and speed, and presentiment in the same perceptual task.
In the former, the task involves identifying a schematic face in the middle of a series of rapidly presented (100 ms) white noise images. Both EEG and behavioural measures (subject image determinations) are recorded, because the EEG often presents a neural signature when the face is shown, as compared to when the face is absent. Subjects whose EEG signature indicates an image are more likely to correctly identify the presence of an image, if it is actually present. Similarly, subjects whose EEG patterns suggest no face, are less likely to claim there is a face, if no face is present (false call). True hits (when the subject determination of a face is in agreement with the presence of a face) and true misses (when the subject determination of no face is in agreement with the actual absence of a face) are compared between random sources: in several experiments, there remarkable differences in speed of identification and accuracy have been recorded, with superior performance from quantum-generated stimuli rather than pseudo-randomly generated stimuli. These findings have significant bearing on the role of consciousness on quantum collapse, favouring Wigner’s interpretation that consciousness is a requisite for collapse.17
In presentiment research, a prediction of the upcoming stimulus (face or no face) is made by analyzing the pre-stimulus baseline region of the EEG, which reveals differences between the two categories. The group has consistently found a prediction accuracy in the 53%-55% range, only marginally above the 50% chance level, but hugely significant statistically given the large number of trials.
These studies are blended into mainstream psychology work, for example, the effect of coffee on perceptual accuracy. As such, they provide a potential template for psychology researchers to use, a means to add psi research into their programs without too much controversy.
Replication
Dick Bierman is moving towards developing multi-lab collaborations with sceptical or ‘adversarial’ groups in replicating the implicit precognition effect revealed in innovative research by Daryl Bem in the US.
Additionally, Bierman18 has developed a theory called Consciousness Induced Restoration of Time Symmetry (CIRTS), which attempts to explain precognition/presentiment as a function of the conscious brain’s attempt to restore time symmetry. In our observed universe time flows in one direction, from present to the past, and hence is unsymmetrical. Bierman’s theory allows for precognition-type effects, where time can flow from the future to the present, restoring symmetry. Bierman’s work is an important milestone in approaching these phenomena, termed ‘advanced waves’ by physicists, in a theoretically rigorous manner.
Russia
Scientific psi research in Russia began around 1870 with a commission headed by DI Mendeleev investigating mediums. A coordinated Soviet program began in 1924, with the forming by the government of the Russian Committee for Psychical Research. These studies and programs collapsed in the turmoil that started in the mid-1930s. The government renewed its interest in unconventional research in the 1960s. In 1973, a state-sponsored report determined that psi phenomena are genuine while the causes remain unknown.
A 1994 survey found that the main focus of psi research in Russia was on psychokinesis studies, with experiments using a variety of physical systems such as diodes and oscillators, and biological systems from micro-organisms and plants to human physiology, many reporting statistically significant results.19
PK studies have been carried out in prestigious institutions such as Moscow State University (Biology and Psychology Departments), St Petersburg State University (Biology Department), Simferopol State University (Division of Animal and Human Physiology) and research institutes of the Russian and Ukrainian Academies of Sciences and the Russian Academy of Medical Sciences.
According to one, relatively conservative estimate, in the mid-1980s, psi research in the Soviet Union in the 1980s was being carried out in around forty institutes and laboratories involving 400 individuals. Other estimates go as high as several thousand researchers and support staff.
See also Psi Research in Russia.
Sweden
Swedish Society for Parapsychological Research (SSPR).
The SSPR was formed in 1948 to study of phenomena described as psychic or meta-psychological, conducted under effective controls and with a balanced critical examination of phenomena and experimental results.
Adrian Parker
Adrian Parker, a professor at the Göteborgs Universitet (University of Gothenburg), researches ESP in the ganzfeld, also exceptional experiences among twins, where his findings suggest that those identical twins who were late splitters (who shared membranes and placentas, medically defined as ‘monochorionic-monoamnionic’) are more likely to demonstrate telepathy effects in controlled sessions, supporting an entanglement-like hypothesis for psychic twin experiences.20
Etzel Cardeña
Etzel Cardeña, a Mexican psychologist at Lunds Universitet (Lund University, Sweden) heads Sweden’s single research chair that includes parapsychology, the Centre for Research on Consciousness and Anomalous Psychology (CERCAP). He has held the post since 2003. The Centre’s approach resembles that of the Koestler Parapsychology Unit during the tenure of Robert Morris: research into parapsychological phenomena as part of a wider investigation of unusual experiences.
Hypnotism is a central element in Cardeña’s psi research, revealing significant relationships between success in ganzfeld experiments achieved with hypnotized subjects and the degree of change of consciousness they report during the session, their degree of belief of personal success in the experiment, and their previous psi experiences.21
Others
Denmark
The Danish Society for Psychical Research, founded in 1905, is the third oldest of its kind in the world. Its purpose is to examine without prejudice or prepossession, and in a scientific spirit, apparently inexplicable phenomena such as ESP.
Hungary
The Open University Szintezis (OUS) was established in 1997 by philosopher Ervin Laszlo. This free University, directed by Zoltán Vassy, aims to address essential questions of human existence, including the nature of ESP, with formal courses and a laboratory for psi experiments.
Iceland
Erlendur Haraldsson (1931–2020) of the University of Iceland in Reykjavik, conducted surveys on psychical experiences and beliefs, and alleged encounters with the dead; conducted precognition and ganzfeld experiments; investigated cases of children who claim past-life memories and carried out psychological studies. He published over seventy papers in peer-reviewed journals, and numerous books.
European Psi Research Table
|
Institution |
Leading researcher(s) |
Description of current or usual research activities |
|
University of Groningen(NL) |
Dick Bierman, Jacob Jolij |
Presentiment, Mind-matter interaction. Developing theoretical frameworks for psi. |
|
IGPP (Germany) |
Wolfgang Ambach, Jiri Wackerman, Dieter Vaitl, Harold Atmanspacher |
Theoretical models, concealed information tests, distant EEG correlations. |
|
University of Freiberg (Germany) |
Stefan Schmidt |
Meta-analyses, healing and meditation research. Associate of the Samueli Institute. |
|
University of Munich (Germany) |
Markus Maier |
Implicit precognition, mind-matter interaction in special populations. |
|
Institut Métapsychique International – IMI (France). |
Mario Varvoglis |
Presentiment, telepathy, including the new Sharefeld technique. General promotion of psi research. |
|
University of Gothenburg (Sweden) |
Adrian Parker |
Ganzfeld, twin research. |
|
University of Lund – CERCAP (Sweden) |
Etzel Cardeña |
Hypnotism and psi, Ganzfeld. |
|
University of Iceland |
Erlendur Haraldsson (deceased) |
Surveys on paranormal experiences: reincarnation type, mediumship |
|
University of Padova |
Patrizio Tressoldi |
Presentiment, EEG correlations, meta-analyses. |
Michael Duggan
Literature
Ambach, W. (2008). Correlations between the EEGs of two spatially separated subjects: a replication study. European Journal of Parapsychology 23/2, 131-46.
Atmanspacher H. & Fach W. (2013). A Structural-phenomenological typology of mind-matter correlations. Journal of Analytical Psychology 58, 219-44.
Atmanspacher, H. & Fach, W. (2014). Mind-matter correlations in dual-aspect monism according to Pauli and Jung. In Beyond Physicalism ed by E. Kelly & A. Crabtree. Lanham, Maryland, USA: Rowman and Littlefield.
Batthyany, A. (2010). Retrocausal habituation and induction of boredom: A successful replication of Bem (2010; Studies 5 and 7) (27 November). [Available online at SSRN.]
Batthyany, A., Kranza, G.S., & Erber, A. (2009). Moderating factors in precognitive habituation: The roles of situational vigilance, emotional reactivity, and affect regulation. Journal of Society for Psychical Research 73, 65-82.
Bem, D., Tressoldi, P., Rabeyron, T., & Duggan, M. (2015). Feeling the future: A meta-analysis of 90 experiments on the anomalous anticipation of random future events. [Published online at F1000Research.]
Bierman, D.J. (1998). The effects of THC and psilocybin on paranormal phenomena. [Paper presented at: Psychoactivity: A Multidisciplinary Conference on Plants, Shamanism, and States of Consciousness, Amsterdam.]
Bierman, D.J. (2010). Consciousness-induced restoration of time symmetry (CIRTS): A psychophysical theoretical perspective. Journal of Parapsychology 24, 273-300.
Jolij, J., & Bierman, D. (2017). Implicit psi in a stimulus detection task: can uPK and precognition affect perceptual performance? [Paper presented at the 60th Annual Convention of the Parapsychological Association in Athens, July 2017.]
Bӧsch, H., Steinkamp, F., & Boller, E. (2006). Examining psychokinesis: The interaction of human intention with random number generators—A meta-analysis. Psychological Bulletin 132, 497-523.
Derakhshani, M. (2015). On the statistical replicability of Ganzfeld studies. In Parapsychology, A Handbook for the 21st Century ed by E. Cardeña, J. Palmer & D. Marcusson-Clavertz. Jefferson, North Carolina, USA: McFarland & Co.
Ertel, S. (1988). Raising the hurdle for the athletes’ Mars effect: Association co-varies with eminence. Journal of Scientific Exploration 2, 53-82.
Ertel, S. (1996). Space weather and revolutions: Chizhevsky's heliobiological claim scrutinized. Studia Psychologica 38, 3-22.
Ertel, S. (1997). Bursts of creativity and aberrant sunspot cycles. In The Scientific Study of Human Nature: Tribute to Hans J. Eysenck at Eighty ed. by H. Nyborg. Oxford: Elsevier, 491-510.
Ertel, S. (2014). Psi ability assessment through the Ball Selection Test. A challenge for psychometrics. In Evidence for Psi ed. by D. Broderick and B. Goertzel. efferson, North Carolina, USA: McFarland & Co.
Fach, W., Atmanspacher, H., Landolt, K., Wyss, T. & Rössler, W. (2013). A comparative study of exceptional experiences of clients seeking advice and of subjects in an ordinary population. Frontiers of Psychology 4, 65.
Jensen, C.G., & Parker, A. (2012). Entangled in the womb? A pilot study on the possible physiological connectedness between identical twins with different embryonic backgrounds. Explore: The Journal of Science and Healing 8/6, 339-47.
Kramer, W.H. Bauer E., & Hövelmann G.H. (eds.) (2012). Perspectives of Clinical Parapsychology. An Introductory Reader. Utrecht: Stichting Het Johan Borgmans Fonds.
Marcusson-Clavertz, D., & Cardeña, E. (2011). Hypnotizability, alterations in consciousness, and other variables as predictors of performance in a ganzfeld psi task. Journal of Parapsychology 75/2, 235-59.
Maier M.A., Büchner, V.L., Kuhbandner, C., Pflitsch, M., Fernández-Capo, M., & Gámiz-Sanfeliu, M. (2014). Feeling the future again: Retroactive avoidance of negative stimuli. Journal of Consciousness Studies 21/9-10), 121-52.
Mossbridge, J., Tressoldi, P.E., & Utts, J. (2012). Predictive physiological anticipation preceding seemingly unpredictable stimuli: A meta-analysis. Frontiers in Psychology 3, 390.
Mossbridge, J.A., Tressoldi, P., Utts, J., Ives, J.A., Radin, D., & Jonas W.B. (2014). Predicting the unpredictable: critical analysis and practical implications of predictive anticipatory activity. Frontiers in Human Neuroscience 8, 146.
Schmidt, S. (2012). Can we help just by good intentions? A meta-analysis of experiments on distant intention effects. Journal of Alternative and Complementary Medicine 18/6, 529-33.
Schmidt, S., Schneider, R., Utts, J., & Walach, H. (2004). Distant intentionality and the feeling of being stared at: Two meta-analyses. British Journal of Psychology 95/2, 235-47.
Storm, L., Tressoldi, P.E., & Di Risio, L. (2010). Meta-analyses of free-response studies 1992-2008, assessing the noise reduction model in parapsychology. Psychological Bulletin 136, 491-4.
Storm, L., Tressoldi, P.E., & Di Risio, L. (2012) Meta-analysis of ESP studies, 1987–2010: Assessing the success of the forced-choice design in parapsychology. Journal of Parapsychology 76/2, 243-74.
Subbotsky, E. (2013). Sensing the future: Reversed causality or a non-standard observer effect? The Open Psychology Journal 6, 81-93.
Tressoldi, P.E. (2011). Extraordinary claims require extraordinary evidence: The case of non-local perception, a classical and Bayesian review of evidences. Frontiers in Psychology 2, 117.
Tressoldi, P.E., Martinelli, M., Semenzato, L., & Cappato, S. (2011). Let your eyes predict: Prediction accuracy of pupillary responses to random alerting and neutral sounds. SAGE Open 1/2, 1-7.
Tressoldi P.E., Martinelli, M., & Semenzato, L. (2014a) Pupil dilation prediction of random events [v2; ref status: approved with reservations 2]. [Published online at F1000Research.]
Tressoldi, P.E., Pederzoli, L., Caini, P., Fedele, P., Ferrini, A., & Melloni, S. (2014b). Brain-to-brain (mind-to-mind) interaction at distance: A pilot study. [Available online at SSRN.]
Tressoldi, P.E., Pederzoli, L., Caini, P., Ferrini, A., Melloni, S., Richeldi, D., & Richeldi, F. (2014c). Mind-Matter Interaction at a Distance of 190 km: Effects on a Random Event Generator Using a Cutoff Method (April 14, 2014). NeuroQuantology 2014 3, 337-43.
Varvoglis, M.P., Bancel, P., Ahmed, D., Bailly, J. & Béguian, C. (2013). The Sharefield: a novel approach for forced-choice GESP research. [Paper presented at the 56th Parapsychological Convention, Viterbo, Italy.]
Vilenskaya, L. & May, E., (1994). Some aspects of parapsychological research in the former Soviet Union. Subtle Energies 3, 1-24
Walach, H., & von Stillfried, N. (2011). Generalised quantum theory—basic idea and general intuition: a background story and overview. Axiomathes 21/2, 185-209.
Endnotes
- 1. Batthyány (2010); Batthyány et al (2009).
- 2. Ambach (2008).
- 3. Atmanspacher & Fach (2013, 2014).
- 4. Bӧsch, Steinkamp, & Boller (2006).
- 5. Fach et al (2012); Kramer, Bauer & Hövelmann (2012).
- 6. Ertel (1988, 1996, 1997).
- 7. Ertel (2014).
- 8. Maier et al (2014).
- 9. Walach and Stillfried (2011).
- 10. Schmidt et al. (2004); Schmidt (2012).
- 11. Varvoglis et al (2013).
- 12. Storm, Tressoldi and Di Risio (2012); Mossbridge, Tressoldi and Utts (2012); Bem, Tressoldi, Rabeyron & Duggan (2015).
- 13. Tressoldi et al (2011); Tressoldi et al (2014a, 2014b, 2014c).
- 14. Tressoldi et al (2014b).
- 15. Mossbridge, Tressoldi and Utts
- 16. Mossbridge et al (2014).
- 17. Bierman and Jolij (2017).
- 18. Bierman (2010).
- 19. Vilenskaya & May (1994).
- 20. Jensen & Parker (2012).
- 21. Marcusson-Clavertz & Cardeña (2011).