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 3,367 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, 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.

The study is implemented as a natural field experiment embedded in an environmental organization’s first fundraising campaign following the public announcement of the approval of a climate law through a referendum. The target population consists of individuals on the organization’s contact list, including both potential donors and past donors. A total of 3,367 individuals are included in the randomized sample.

The experiment follows a two-by-two factorial design. The first treatment dimension varies the motivational appeal of the donation request. In the warm-glow condition, the message emphasizes expressive participation and the symbolic value of contributing to a successful civic movement, highlighting the collective achievement of the referendum. In the instrumental condition, the message emphasizes the concrete role of donations in financing follow-up activities and supporting effective implementation of the newly approved law, thereby stressing the tangible impact of contributions.

The second treatment dimension varies the spatial scope of benefits associated with the recently approved climate law. In the local emphasis condition, the donation request highlights benefits for the jurisdiction in which the referendum took place. In the global emphasis condition, the message highlights global climate benefits. Given the characteristics of climate protection, the local and global versions are constructed to describe comparable mitigation and co-benefit outcomes, such as reductions in carbon emissions and associated health improvements, using closely aligned language. The primary difference across conditions is the geographic scope of beneficiaries rather than the type of benefit described.

Randomization is conducted at the individual level using Stata with a fixed random seed. Assignment is stratified by nationality, past donation category, and past engagement with the organization. Past donation is categorized into three groups: no recorded past donation, low past donation, and high past donation, based on the distribution of recorded donation amounts among past donors. Engagement is categorized into low and high engagement based on the median of a pre-treatment engagement measure. Stratification ensures balanced assignment across treatment arms along these dimensions.

Each individual receives exactly one version of the donation request through the organization’s standard communication channels. There is no self-selection into treatment assignment, as the subject line and initial receipt of the message are treatment independent. All analyses are conducted within an intention-to-treat framework based on initial assignment.

The primary analysis estimates the average effects of motivational appeal and spatial scope emphasis on donation behavior, as well as their interaction. Main effects are estimated pooling across the complementary treatment dimension to maximize statistical power. The interaction between motivational appeal and spatial scope captures whether sensitivity to the geographic diffusion of benefits differs between warm-glow and instrumental appeals.

In addition to the factorial treatment effects, the study examines heterogeneity across pre-treatment characteristics, including nationality, past donation category, and past engagement with the organization. These analyses assess whether responses to motivational appeal and spatial scope vary systematically across donor history, engagement levels, and nationality.

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 3,367 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.

This study is a natural field experiment implemented during the environmental organization’s first fundraising campaign following the public announcement of the approval of a Climate Law through a referendum. The Climate Law has already been passed and is legally binding at the time of the intervention. Donations therefore cannot influence adoption of the law. The setting constitutes a post-provision environment in which the public good has already been secured.

The sample consists of approximately 3,367 individuals in the organization’s contact database, including previous donors as well as individuals who have interacted with the organization but have never donated. The unit of randomization is the individual. Each participant receives one electronic message inviting them to donate.

The subject line, header, layout, and general structure of the message are identical across treatment arms. Treatment variation is limited to selected sentences within the body of the message. Treatment assignment is not visible to participants, and individuals do not choose which message they receive.

Treatment design

The experiment follows a two-by-two factorial design.

Motivational appeal (Warm Glow vs Instrumental)

In the Warm Glow condition, the message emphasizes expressive participation and the symbolic value of contributing to a successful civic movement, highlighting the collective achievement of the referendum.

In the Instrumental condition, the message emphasizes the concrete role of donations in financing follow-up activities and supporting effective implementation of the Climate Law, stressing the tangible impact of contributions.

Spatial scope of benefits (Local vs Global)

In the Local emphasis condition, the message highlights benefits of the Climate Law for the jurisdiction in which the referendum took place, including local environmental improvements and associated co-benefits.

In the Global emphasis condition, the message highlights global climate benefits, including contributions to emissions reductions and broader climate mitigation outcomes.

The local and global versions describe comparable mitigation and co-benefits using closely aligned language. The primary difference across conditions is the geographic scope of beneficiaries.

This yields four treatment cells: Warm Glow Local, Warm Glow Global, Instrumental Local, and Instrumental Global.

Randomization and stratification

Randomization is conducted at the individual level using Stata with a fixed random seed. Assignment is stratified by nationality, prior donation status, and prior engagement with the organization to ensure balance across treatment arms.

Heterogeneity analyses

Heterogeneous treatment effects are estimated using interaction terms between treatment assignment and pre-treatment characteristics. Pre-specified heterogeneity analyses focus on nationality, prior donation status, and prior engagement.

ITT and per-protocol analyses

All primary analyses are conducted within an intention-to-treat framework based on initial assignment. Because the subject line and message headers are treatment independent, exposure to the campaign at the email-opening stage is not influenced by treatment assignment. This allows for complementary per-protocol analyses, which are reported restricting the sample to individuals who open the email or engage with the campaign.

Randomization is conducted at the individual level. Each of the 3,367 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;

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

3367 individuals

3367 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 3,367 individuals.

The study is individually randomized with no clustering. The planned sample size is 3,367 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.