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How to hire helpers? Evidence from a field experiment

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How to hire voluntary helpers? We shed new light on this question by reporting a field experiment in which we invited 2859 students to help at the ‘ESA Europe 2012’ conference. Invitation emails varied non-monetary and monetary incentives to convince subjects to offer help. Students could apply to help at the conference and, if so, also specify the working time they wanted to provide. Just asking subjects to volunteer or offering them a certificate turned out to be significantly more motivating than mentioning that the regular conference fee would be waived for helpers. By means of an online-survey experiment, we find that intrinsic motivation to help is likely to have been crowded out by mentioning the waived fee. Increasing monetary incentives by varying hourly wages of 1, 5, and 10 Euros shows positive effects on the number of applications and on the working time offered. However, when comparing these results with treatments without any monetary compensation, the number of applications could not be increased by offering money and may even be reduced.

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  1. The conference took place from September 12th-15th, 2012. Conference website:

  2. Different from Al-Ubaydli and Lee (2011), we vary the content of the invitation while keeping its format constant.

  3. See Kosfeld and Neckermann (2011) and Bradler et al. (2013) for other studies on the impact of certificates.

  4. The standard hourly wage for a student assistant is 8.80 Euros at the University of Cologne. We chose the 10 Euros offer since this is a wage for a typical well-paid student job in Cologne.

  5. It might well be that working for a price that is lower than the opportunity costs of work leads to a negative self-image and thereby reduces labor supply on top of the mere economic rationale.

  6. The emails were signed by two professors of the faculty of Management, Economics and Social sciences. The full text can be found in the appendix. The original letters were sent out in German. The German texts are available from the authors upon request.

  7. Before setting up the conference it was not fully clear how many helpers we would actually need. One main task of the helpers was to be present and providing assistance at several parallel conference sessions. In total we had 56 sessions of one and a half hours each. Thus, we needed roughly 86 hours of work for this task. After we received the applications, we hired the applicants from the 10 Euros treatment and paid them accordingly. The authors are aware of the fact that working for a hourly wage might not be considered as “volunteering” and might create cognitive dissonance for potential applicants. In order to keep the invitations as similar as possible between the treatments, we chose to stick to this wording in the invitation email.

  8. We constructed a dummy variable to classify the gender of each subject according to the first name (in order to recruit helpers for the conference, the central office of the university provided us a simple database containing individual entries about (i) first name, (ii) surname and (iii) email address of the students). The resulting pool of subjects is made up of 44 % of females, 49 % of males and a remaining 7 % classified as gender “missing” since it was not possible to establish with certainty the correct gender of these subjects based on their first name.

  9. Before the first mailing, the authors committed to recruit applicants from the treatment with the highest number of applications. The first mailing was sent out on July 6th, followed by two reminders containing the same information on July 26th and August 2nd.

  10. We treat every subject to whom we sent an email but who did not apply as a subject offering zero minutes of work.

  11. The same results hold when we only compare the minutes offered by subjects that applied for help (\(p=.622\), MWU-test, two-sided). The sample size of two in Waived Fee is too small for any statistical test based on this measure.

  12. Only taking into account the number of minutes of those subjects who actually applied for help confirms the observation of an increasing labor supply (\(p = .037\), Jonckheere-Terpstra-test, one-sided).

  13. There are several interpretations for such crowding-out effects based on formal economic models. An interpretation of this result in light of Bénabou and Tirole (2006) is that weak monetary incentives reduce the signaling value of an application to demonstrate pro-social preferences. An interpretation in the spirit of Sliwka (2007) is that offering weak monetary incentives may reveal that volunteering without any monetary compensation is not the norm of behavior (hence, money has to be offered to attract applications). At the same time, the hourly wage is too small to attract applications that are driven by purely pecuniary motives.

  14. Interestingly, comparing the total minutes offered between all non-monetary (Voluntary, Waived Fee, Certificate) to the total minutes offered in all monetary treatments, we observe no statistical significant difference considering all participants (\(p=.216\), MWU-test, two-sided) and applicants exclusively (\(p=.156\), MWU-test, two-sided).

  15. Another explanation could be that different types of helpers apply in different treatments. Therefore, after the conference we sent out emails with invitations to incentivized questionnaires asking participants for individual characteristics and personality traits, e.g., the surveys on human values by Schwartz (1992) or motives to volunteer by Clary et al. (1998). Analyzing the data from the questionnaire, no significant difference could be found between the six treatment groups – which may, however, be due to a lack of statistical power as the response rates of subjects who actually applied as a helper was only about 50%.

  16. As is common procedure in the literature on survey design [see, for instance,Costello and Osborne (2005); Acock (2014, Chap. 12)], we consider factors with an Eigenvalue larger than 1.

  17. For our purposes, for instance, we needed 86 hours of help. Given the observed application frequency and the average working time offered, it is necessary to contact at least 190 students (=86/.0252 \(\cdot \) 18 h) in the Voluntary treatment to receive sufficient support in expected terms. When offering money, e.g., in the 10 Euros treatment, we only would have needed an applicant pool of only 85 students (=86/.04 \(\cdot \) 25 h) to satisfy the requirements of the conference.


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We are grateful to Thomas Lauer, Oliver Gürtler, Christian Ruppert and Julia Stauff for their help in conducting the experiment. We also thank the Editor, two anonymous referees, Max Bazerman, Oleg Badunenko, Antonio Filippin, Andrew Kinder, Steve Levitt, John List, Susanne Neckermann, Gerhard Riener, Alessandro Saia, and seminar participants at University of Chicago, University of Paderborn and EALE Conference 2014 (Ljubljana) for helpful comments. Financial support from the Deutsche Forschungsgemeinschaft through grant TP3 Design of Incentive Schemes within Firms: Bonus Systems and Performance Evaluations (sub-project of the DFG-Forschergruppe Design and Behavior) is gratefully acknowledged.

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Correspondence to Tommaso Reggiani.

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1.1 Invitation email

Subject: Volunteers for a conference

Dear 〈Name inserted〉,

From 12th to 15th of September 2012 the conference of the Economic Science Association (ESA) is taking place at the University of Cologne. Over 200 economists from all over the world get together to talk about current findings from the field of behavioural economics. Besides many interesting speeches, there are presentations from international elite researchers (i. a. Max Bazerman from Harvard Business School). You can find more details on the conference homepage For the preparation and procedure of the conference we are looking for volunteers, who support us in the organization before and during the conference. During the times you are not working as a volunteer you can attend interesting lectures and discussions.


{Certificate: At the end of the conference we hand a certificate out to you.}

{Waived Fee: The participation fee which you are exempted from would be about 320 .}

{1 Euro: Per working hour you get 1 .}

{5 Euros: Per working hour you get 5 .}

{10 Euros: Per working hour you get 10 .}

In case we aroused your interest you can apply online and without much expenditure of time under this link. You can also indicate on which days and how many hours you would like to work for us: You can find the application here.

Yours sincerely,

Prof. Dr. Bernd Irlenbusch & Prof. Dr. Dirk Sliwka

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Conrads, J., Irlenbusch, B., Reggiani, T. et al. How to hire helpers? Evidence from a field experiment. Exp Econ 19, 577–594 (2016).

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