# Donor Thank-You Letters: What Two Field Experiments Actually Found

Two preregistered field experiments found that neither handwriting nor a "thank you in advance" line produced a statistically significant change in giving. This article explains what that evidence rules out, how it fits with the research on thank-you calls, and what a letter should say instead.

## Key takeaways
- In a preregistered field experiment with 1,132 past donors of a German opera house, 19.3% gave after a handwritten thank-you postcard and 18.7% after a printed one, a difference that was not statistically significant (Adena, Huck and Neyse, 2025).
- In the same research, among 8,800 appeal letters, response was 1.8% with a closing "Thank you in advance" and 1.6% without it, a difference that was not significant.
- A randomized study of 500,000 new donors at 67 public television stations and a national nonprofit found that thank-you calls made five to seven months after a gift did not change retention (Samek and Longfield, calls placed 2011 to 2016).
- That randomized evidence is stronger than the older, observational call research, so treat the call effects reported elsewhere as a hypothesis to test rather than a promise.
- In Bloomerang's 2026 survey, 94% of donors said they are motivated to give when an organization tells them exactly where their money will go, which makes the content of a thank-you the part worth testing.

A donor thank-you letter does not become more effective by being handwritten. In two preregistered field experiments with a German opera house, covering about 10,000 people, neither a handwritten thank-you postcard nor a "thank you in advance" line significantly changed how many people gave or how much, according to Maja Adena, Steffen Huck, and Levent Neyse, whose letters were mailed in May 2023 and whose results appeared in [Experimental Economics (2025)](https://doi.org/10.1017/eec.2025.10033). In that test, the form of the thank-you did not measurably change the next gift. What the letter says is untested, and it is the part worth testing next: in [Bloomerang's 2026 Giving Signals survey](https://bloomerang.com/news/bloomerang-releases-2026-giving-signals-report) of more than 1,000 active donors, 94% said they are motivated to give when an organization tells them exactly where their money will go. This article covers what the experiments found, how they fit with the research on thank-you calls, and the five lines a thank-you letter should contain.

## What did the thank-you letter experiments find?

Both experiments were preregistered, which means the researchers registered their analysis plan before receiving the data, so they could not search for a favorable result afterward. In the first, the opera house sent 8,800 fundraising letters. Half ended with the phrases "Thank you in advance" and "Thank you for your support", and half did not. Response was 1.6% without the phrases and 1.8% with them, a difference that was not statistically significant, according to the [authors' discussion paper](https://bibliothek.wzb.eu/pdf/2025/ii25-202.pdf) (Table 2).

In the second, 1,132 past donors received a postcard thanking them for their earlier gifts, alongside the opera house's next fundraising letter. For 569 of them, the message was written by hand, one card at a time, with a pen chosen to leave visible pressure marks. The other 563 received the same message printed. Response was 19.3% after the handwritten card and 18.7% after the printed one. Donors who gave after a handwritten card gave about 87 euros on average, against about 114 euros after a printed card. None of these differences was statistically significant, and the authors write that their "zero results are quite precisely estimated", meaning the study was large enough that a meaningful effect would likely have been detected.

**19.3% vs 18.7%** — of past donors gave again after a handwritten thank-you postcard versus a printed one, a difference that was not statistically significant ([Adena, Huck and Neyse (2025), WZB discussion paper, Table 2](https://bibliothek.wzb.eu/pdf/2025/ii25-202.pdf))

The study has limits worth stating. It was run with one arts organization in Germany, and the authors note that thanking norms differ across cultures. It tested the form of the thanks, not its content, and it did not test speed. It is still one of the most rigorous recent tests of a practice that many development offices put real staff time into.

## Does this contradict the research on thank-you calls?

In part, and the disagreement deserves to be stated openly. Our [thank-you call economics](https://donorinsights.com/articles/thank-you-call-economics) article reports a finding from Penelope Burk's donor research of the 2000s that first-time donors who received a thank-you call from a board member within 24 hours made a second gift 39% larger, as summarized by [Neon One](https://neonone.com/resources/blog/calling-to-thank-donors/). A randomized study points the other way. Anya Samek and Chuck Longfield, in [American Economic Journal: Applied Economics](https://www.aeaweb.org/articles?id=10.1257/app.20210068) (2023), randomly assigned 500,000 new donors at public television stations and a national nonprofit to receive a thank-you call or not. Fundraising professionals had predicted the calls would raise retention by 80%. The study found a precisely estimated null effect. This is background research with calls placed from 2011 to 2016, but it is the largest randomized test of thank-you calls available.

The studies differ in ways that matter. In Samek and Longfield's experiment, according to the [American Economic Association's summary](https://www.aeaweb.org/research/charts/charitable-giving-thank-you-calls), callers reached donors five to seven months after the gift, addressed them by name, explained the impact of the gift, and asked for nothing. Repeat giving leveled off at around 40% for called and uncalled donors alike. Burk's calls came from board members within a day. Burk's comparison and other non-randomized call data let organizations choose whom to call, so donors who were called may have differed from those who were not, for example by having a working phone number on file. A randomized design removes that difference, which is why its result carries more weight.

Thank-you research compared by design (figures from the sources linked in this article)

| Study | Design | What was tested | Result |
| --- | --- | --- | --- |
| Adena, Huck and Neyse (2025) | Two preregistered field experiments, about 10,000 people (1,132 in the postcard test) | Handwritten vs printed postcard, and a "thank you in advance" line | No significant difference |
| Samek and Longfield (2023, calls 2011 to 2016) | Randomized field experiment, 500,000 new donors | Thank-you call five to seven months after the gift, no ask | No effect on retention, about 40% repeat in both groups |
| Penelope Burk's research (2000s) | Older donor research, reported through a secondary summary | Board member's call to first-time donors within 24 hours | Second gift 39% larger |

The fair reading is that the strongest evidence available finds no effect from the form of a thank-you, whether a handwritten card, a polite phrase, or a late call that follows standard practice. The positive evidence is older, uses weaker designs, and tested something different: a fast call from a volunteer leader. A call program may still be worth running, but its effect is a hypothesis to test on your own donors, and the call economics article should be read with that caveat.

## What should a donor thank-you letter say?

If the form of the letter did not measurably change giving, its content is the remaining candidate. No randomized study in this article tested letter content, so the five lines below rest on what donors say they want, on the research about identification, and on what the law requires. Bloomerang's finding that 94% of donors are motivated when told exactly where their money goes is stated intent, not measured behavior, and stated intent often runs ahead of action. The recommendations are a starting point to test, not a proven formula.

1. The receipt line. Name the organization, the amount, and the date. The [IRS](https://www.irs.gov/taxtopics/tc506) requires a donor to have a bank record or written communication showing all three for any cash gift, and for a gift of $250 or more, a written acknowledgment that also states whether the donor received any goods or services in return.
2. The specific-outcome line. Say what this gift does, in concrete terms: a number of meals, hours of tutoring, or nights of shelter, not "your support makes a difference".
3. The person line. Describe one person or one moment the program touched, with permission. [A 2025 meta-analysis](https://doi.org/10.1177/08997640251317403) found that identifying with beneficiaries was associated with more giving (r = .24), as our [fundraising psychology roundup](https://donorinsights.com/articles/fundraising-psychology-research) explains.
4. The donor's-role line. Make the donor the subject of a sentence about the outcome: "You gave a second-grader a tutor this fall."
5. The next-update line. Tell the donor when they will hear what happened, and then keep the promise.

Three lines are worth cutting. "Thank you in advance" produced no statistically significant change in response in the field experiment above. Generic praise such as "your generous support makes a difference" states no fact a donor could remember. And a new request inside the thank-you turns it into another appeal. No study here tested that last point, so treat it as a practice judgment rather than a finding.

## What does a donor thank-you letter look like with all five lines?

The sample below is for a fictional organization, Harbor Street Literacy Project, and a fictional $100 gift. It is illustrative, written to show each line in place.

Illustrative annotated thank-you letter (fictional organization and donor)

| Line | Sample text | What it does |
| --- | --- | --- |
| Receipt | Thank you for your gift of $100 to Harbor Street Literacy Project on October 12. | Gives the donor the name, amount, and date for their records |
| Specific outcome | Your gift covers ten hours of one-on-one reading tutoring for a second-grader this fall. | Tells the donor exactly where the money goes |
| Person | Last week, Maya read a full chapter book aloud to her tutor for the first time. | Gives the donor someone to picture |
| Donor's role | You are part of the reason a child in our program will start third grade reading on level. | Makes the donor the subject of the outcome |
| Next update | In January, we will write to tell you how this fall's readers progressed. | Sets a date for the next contact that carries news, not a request |

A gift of $250 or more also needs the acknowledgment statement about goods or services described in IRS Topic 506, and an organization's own counsel or accountant should confirm the exact wording.

## Why does the thank-you letter still matter if its form does not?

The thank-you is the first message nearly every new donor receives. In the [Virtuous 2026 benchmark](https://www.prnewswire.com/news-releases/virtuous-releases-2026-nonprofit-fundraising-benchmark-report-median-gift-up-20-as-donors-who-stay-give-more-often-and-more-generously-302748571.html) of 771 mid-sized and large nonprofits, only 25.84% of first-time donors made a second gift. The experiments above say that making the thank-you look more personal will not change that number. They leave open whether telling donors specifically what their gift did will, which is why the content lines are the part to test. The [thank-you timing guide](https://donorinsights.com/articles/thank-you-timing) covers how quickly to send.

**25.84%** — of first-time donors made a second gift across 771 mid-sized and large nonprofits ([Virtuous, 2026 Nonprofit Fundraising Benchmark Report](https://www.prnewswire.com/news-releases/virtuous-releases-2026-nonprofit-fundraising-benchmark-report-median-gift-up-20-as-donors-who-stay-give-more-often-and-more-generously-302748571.html))

> **How strong is this evidence?**
>
> The two opera-house experiments (run in 2023, published in 2025) are preregistered and randomized, but they come from one organization in one country. The call experiment is randomized and very large, but its calls were placed from 2011 to 2016 and came months after the gift. The survey figure from Bloomerang (2026) measures what donors say, not what they do. None of these studies tested letter content directly.

Because none of the studies tested what a letter says, the useful test is on your own file. Donor Insights reads your contacts and gifts the way a fund reads a portfolio, so the second-gift rate of donors who received the new five-line letter can be compared with last year's first-time donors who received the old one. That comparison is not randomized, so read it as an indication rather than proof. The [methodology](https://donorinsights.com/methodology) starts from your own giving records, and the same [platform](https://donorinsights.com/platform) shows whether a more specific thank-you is followed by more second gifts.

## FAQ

**Do handwritten thank-you notes increase donations?**

Not in the best recent evidence. In a preregistered field experiment with 1,132 past donors, 19.3% gave after a handwritten postcard and 18.7% after a printed one, a difference that was not statistically significant (Adena, Huck and Neyse, 2025).

**Should a donation appeal say thank you in advance?**

Response did not differ significantly in the one field test. Among 8,800 appeal letters, response was 1.8% with a closing "Thank you in advance" and 1.6% without it, a difference that was not statistically significant. The phrase can be kept or cut on taste.

**What should a donor thank-you letter include?**

The organization's name, the amount, and the date, then what the gift specifically does, one person or moment the program touched, the donor as the subject of the outcome, and when the donor will hear more. Gifts of $250 or more also need a statement about whether any goods or services were provided.

**Do thank-you calls work?**

The evidence is mixed. Older donor research associates calls with larger and more frequent giving, but a randomized study of 500,000 new donors found that calls made five to seven months after the gift had no effect on retention. Treat a call program as something to test on your own donors.

**Is a thank-you letter legally required?**

For a single gift of $250 or more, the donor needs a written acknowledgment from the organization to claim a deduction, stating the amount and whether any goods or services were provided. For smaller cash gifts, a bank record or a written communication showing the organization, amount, and date is enough. Confirm details with a tax professional.

## Sources
- [Chapman, C. M., Spence, J. L., Hornsey, M. J., & Dixon, L. (2025). Social Identification and Charitable Giving: A Systematic Review and Meta-Analysis. Nonprofit and Voluntary Sector Quarterly, 54(6), 1308-1341.](https://doi.org/10.1177/08997640251317403)
- [Neon One. Calling to Thank Donors (summary of Penelope Burk's research on board thank-you calls to first-time donors).](https://neonone.com/resources/blog/calling-to-thank-donors/)
- [Adena, M., Huck, S., & Neyse, L. (2025). Gratitude in fundraising: Do 'thank you in advance' and handwritten thank you notes impact fundraising success? Experimental Economics, 28(4), 900-909.](https://doi.org/10.1017/eec.2025.10033)
- [Adena, M., Huck, S., & Neyse, L. (2025). Gratitude in fundraising. WZB Discussion Paper SP II 2025-202 (Table 2 response rates and gift sizes).](https://bibliothek.wzb.eu/pdf/2025/ii25-202.pdf)
- [Samek, A., & Longfield, C. (2023). Do Thank-You Calls Increase Charitable Giving? Expert Forecasts and Field Experimental Evidence. American Economic Journal: Applied Economics, 15(2), 103-24. Calls placed 2011-2016.](https://www.aeaweb.org/articles?id=10.1257/app.20210068)
- [American Economic Association (2023). Thank-you calls and donor retention, research summary of Samek and Longfield.](https://www.aeaweb.org/research/charts/charitable-giving-thank-you-calls)
- [Bloomerang with The Harris Poll (2026). 2026 Giving Signals Report, release. 1,000+ active U.S. donors, fieldwork March 13-24, 2026.](https://bloomerang.com/news/bloomerang-releases-2026-giving-signals-report)
- [Virtuous (2026). 2026 Nonprofit Fundraising Benchmark Report, release. 771 mid-sized and large U.S. nonprofits, 2025 data.](https://www.prnewswire.com/news-releases/virtuous-releases-2026-nonprofit-fundraising-benchmark-report-median-gift-up-20-as-donors-who-stay-give-more-often-and-more-generously-302748571.html)
- [Internal Revenue Service. Topic no. 506, Charitable contributions (reviewed September 2026).](https://www.irs.gov/taxtopics/tc506)

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