Table 2. Between-group comparisons.

			DG	UG	UG-DG	MAO	TG trustor	TG trustee
Student bias
(A + B)	vs	(C + D)	−0.060*	0.008	0.054**	−0.039	−0.168	−0.083
A	vs	C	−0.031	0.021	0.061**	−0.002	−0.093	−0.130
B	vs	D	−0.068	−0.007	0.047	−0.079	−0.242	−0.034
Self-selection bias
(A + C)	vs	(B + D)	0.040	0.023	−0.010	0.020	0.197*	0.240**
A	vs	B	0.051	0.044*	0.000	0.078	0.309	0.170
C	vs	D	0.037	0.017	−0.013	0.001	0.159	0.266**
Subject-pool bias
A	vs	(B + C + D)	−0.012	0.033*	0.039	0.021	0.080	0.049
A	vs	D	−0.017	0.038	0.047*	−0.002	0.067	0.136

Notes: Letters A, B, C and D refer to the groups depicted in Figure 1a. Group A denotes students, volunteers; B students, non-volunteers; C non-students, volunteers; D non-students, non-volunteers. (A + B) correspond to all students (volunteers and non-volunteers); (C + D) to all non-students (volunteers and non-volunteers); (A + C) to all volunteers (students and non-students); (B + D) to all non-volunteers (students and non-students). Lastly (B + C + D) correspond to the sum of the subject pool except students volunteers. *, ** indicate significance at the 0.10, and 0.05 levels, respectively. Comparisons based on Wald tests from models of Table 1.