Warm Glow, Scope and Parochialism: A Natural Field Experiment After the Climate Law Referendum

The intervention is a natural field experiment embedded in the environmental organization’s first fundraising campaign following the approval of a new climate law through a referendum. A total of approximately 3,543 individuals from the organization’s contact list, including both past and potential donors, receive one of four versions of a donation request. The experiment follows a two-by-two design.

The first treatment dimension varies the motivational appeal of the message. In the warm-glow condition, the appeal emphasizes expressive participation and the symbolic value of being part of a successful civic movement, highlighting the collective achievement of the referendum. In the instrumental condition, the appeal emphasizes the concrete role of donations in financing follow-up activities and ensuring effective implementation of the law, thereby stressing the tangible impact of contributions.

The second treatment dimension varies the spatial scope of benefits emphasized in the appeal. Half of the messages focus on local benefits for Hamburg, while the other half highlight global climate benefits.

Randomization is conducted at the individual level and stratified by nationality, past donation status, and prior engagement with the organization, and time of subscription with regards to the laws passing, ensuring balanced assignment across treatment groups. Each individual receives only one version of the fundraising message through the organization’s standard communication channels.

The intervention is designed to examine why individuals donate after the successful provision of a public good, distinguishing between warm-glow and instrumental motivations and testing for parochial preferences and sensitivity to the spatial scope of benefits through variation in local versus global emphasis.

2026-02-23

2026-02-28

Donation amount (conditional and unconditional), Donation (dichotomous decision variable),

Secondary outcomes include engagement with the donation request, measured by the number of clicks on campaign-related links during the fundraising period, and deregistration from the mailing list, measured by an indicator equal to one if an individual requests removal from the organization’s contact list following receipt of the email. These measures allow us to assess whether motivational appeal and spatial scope emphasis affect not only donation behavior but also broader engagement and potential disengagement from the organization.

The study is a natural field experiment conducted during an environmental organization’s first fundraising campaign following the public announcement of the approval of a climate law through a referendum. Approximately 3,543 individuals from the organization’s contact list are randomly assigned to one of four treatment conditions in a two-by-two design. These conditions vary the motivational appeal of the donation request, contrasting a warm-glow appeal emphasizing expressive participation and symbolic support with an instrumental appeal emphasizing the role of donations in supporting implementation of the law, and vary whether the benefits of the climate law are described in terms of local or global impact.

Not available

Randomization is conducted at the individual level. Each of the 3,543 individuals in the study sample is assigned to one of four treatment conditions in a two-by-two factorial design. Assignment is implemented in Stata using a reproducible random-number seed to ensure full replicability.

Randomization is stratified on two pre-treatment characteristics recorded in the organization’s administrative data:

Past donation status: whether the individual has ever made a prior donation to the organization;
Nationality: International or German
Time of subscription: If potential donow subscribed before or after the laws adoption.
Past engagement: whether the individual has previously interacted with the organization’s communications (e.g., opened or clicked prior messages, or participated in previous actions).

Stratification on these variables ensures balanced assignment across groups that differ in historical involvement with the organization.

Participants do not choose their treatment assignment, and the assignment is not visible to them at any stage. All individuals receive a fundraising message with an identical header, format, and general structure. Only selected lines in the body of the message vary according to the experimental condition. This design eliminates self-selection into treatment and supports unbiased estimation of intent-to-treat effects.

Within each stratum, individuals are assigned with equal probability to one of the four treatment cells (Anonymous–Local, Anonymous–Global, Public–Local, Public–Global). No cluster-level randomization is used.

individuals

No

3,543 individuals

3,543 individuals

Approximately equal numbers of individuals are assigned to each of the four treatment arms, with about 840 to 844 individuals per arm, for a total sample size of 3,543 individuals.

The study is individually randomized with no clustering. The planned sample size is 3,543 individuals allocated approximately equally across four arms in a two-by-two design. The primary analyses estimate the main effects of motivational appeal (warm-glow versus instrumental) and spatial scope (local versus global), as well as their interaction. For the estimation of main effects, observations are pooled across the complementary treatment dimension, yielding approximately half the sample in each main comparison arm. A formal minimum detectable effect size is not pre specified because it depends on the baseline level and variance of the main outcomes in this fundraising campaign, in particular the donation rate, the distribution of donation amounts, and the variance of link click behavior.