Before starting the study, a scoping review was conducted to have a good understanding of the existing research on humor appeal in health promotion materials. A total of 17 eligible articles were identified from the PubMed database (search query: (humor[Title/Abstract] OR humour[Title/Abstract] OR humorous[Title/Abstract]) AND (messages[Title/Abstract] OR advertisements[Title/Abstract] OR ads[Title/Abstract]) NOT (patients[Title/Abstract]) AND English [Language]) or reference lists of relevant articles. Table 1 summarizes the 17 experimental studies on the effectiveness of humor appeal applied to health promotion materials [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]. Most of the studies focused on the effect of humor in video clips targeting young people [14,15,16, 18,19,20,21,22,23,24]. Only 4 studies examined the effect of humor in print materials which aimed to prevent risky behaviors among undergraduate students [10,11,12,13]. We found no studies investigating the effectiveness of humor appeal in print materials targeting the general adult population. The present study therefore intended to answer this research question.

Results for the effectiveness of humor appeal in health promotion materials have been inconsistent across studies. In addition to the differences in target and topic, the type of humor may contribute to the discrepancy. Humor is classified into two major types—playful humor and aggressive humor [27, 28]. Public perception and attitude towards humor often depends on the type of humor. Our previous study suggested that playful humor is more suitable for public health communication because aggressive humor can be controvertial [8]. The present study therefore used playful humor rather than aggressive humor to create the test objects.

We prepared total 17 printable posters with different frames (loss- vs. gain-framed) × tones (humorous vs. non-humorous) × topics (advance care planning, cancer screening, donor registry, smoking cessation, and physical activity) as the test objects. The posters used in this study were shown in Appendix. Each poster consisted of one main message statement and one related illustration, which were placed in accordance with the CDC Clear Communication Index [29].

The main message statements were 22–24 characters long in Japanese as follows. Advance care planning is ‘I want to be myself until the end of my life… So, let’s talk about advance care planning’. Cancer screening is ‘Cancer may creep up on you unawares… So, let’s have cancer screening’. Donor registry is ‘There are people waiting for your help… So, let’s register for the Bone Marrow Donor Program’. Smoking cessation is ‘Tobacco is full of toxins. Let's try to quit smoking together’. Physical activity is ‘Sedentary lifestyle isn't good for your health. Let's move around and refresh your body’. Each message consisted of the consequences (loss) of not taking action in the first half and the benefits (gain) of taking action in the second half. When emphasizing the first half of the message, it was referred to as loss-framed, while when emphasizing the second half of the message, it was referred to as gain-framed [30].

Humor in this study was defined as what provides amusement and provokes laughter. In contrast to prior studies (Table 1), humor was non-verbally expressed by illustrations, because printable posters require the ability to convey a message at a glance. The illustrations were originally painted by one illustrator who fully understood the purposes of the posters. The consequences of not taking action (in loss-framed posters) or the benefits of taking action (in gain-framed posters) were illustrated with humor.

We conducted two web-based surveys among Japanese adults aged 25–64 years in September–November 2022. In the Survey1, total 9 posters for 3 health topics (advance care planning, cancer screening, and donor registry) were evaluated. Loss- and gain-framed humorous posters were compared with a non-humorous poster (as a reference) for each topic. As described later, most people have heard of cancer screening and donor registry at some point, whereas advance care planning is a little-known health topic. The result of Survey1 suggested that humor appeal may not be effective but rather harmful when dealing with a well-known health topic. In order to examine whether this result is consistent and reproducible, we conducted an additional survey. In the Survey2, total 8 posters for 2 common health promotion topics (smoking cessation and physical activity) were evaluated. To make it more detailed, the posters were prepared for a 2 × 2 factorial design (loss-framed humorous, loss-framed non-humorous, gain-framed humorous, and gain-framed non-humorous); target audience were limited to young to middle-aged male office workers; and raters covered both those who were engaged in the risky behavior (i.e. high-involvement) and those who were not (i.e. low-involvement). In Japan, men have a higher smoking rate (27.1% vs. 7.6% in 2019) [31] and a higher mortality of cardiovascular disease (193.8 vs. 110.2, per 100,000 population in 2021) [32] than women. Young to middle-aged men are an important group for lifestyle interventions, [31 and are a suitable audience to receive persuasive messages about smoking cessation and physical activity.

The study protocol was approved by the ethics committees of the Graduate School of Information and Communication (2022–015) and has been conducted in accordance with the Ethical Guidelines for Medical and Biological Research Involving Human Subjects by the Japanese Government.

Participants in the surveys were recruited from an online research panel of a leading research company in Japan (Rakuten Insight Inc., Tokyo, Japan). Recruitment emails were sent to randomly selected eligible registrants (Survey1: men and women aged 25–64 years; Survey2: male office workers aged 25–64 years). Participants with serious illness and medical professionals were excluded through a prescreening process. Applicants for participation in the survey were accepted in the order of receipt until the number of participants reached the quotas (Survey1: 100 men and 100 women per poster; Survey2: 100 male office workers with and without the risky behavior, respectively per poster). All participants voluntarily agreed to participate in the survey after reading a description of the purpose and procedure of the survey. Consent to participate was implied by the completion and submission of the survey.

Eligible participants were randomly assigned one of the posters. They were asked to see a given poster for at least 15 s and rate it in terms of emotional response, comprehensibility, persuasiveness, resistance, and attention attraction. The online questionnaire forms presented the questions one after one through the operation of a “Next” button. Participants answered one question per page and were unable to go back to the previous page. The components of the questionnaire relevant to this study are detailed below.

Participants were asked to what extent the message made them feel 1) surprised, 2) funny 3) fearful, 4) amusing, 5) sad, 6) happy, 7) angry, and 8) unpleasant. Response options were from 1 (not at all) to 5 (extremely) [33, 34]. Because of the lack of established methods of measuring humor perception, humor perception was determined as the average of the amusing and funny scores.

Participants were asked whether the message was easily understandable. Response options were from 1 (strongly disagree) to 5 (strongly agree). Since humor appeal is expected to encourage the peripheral route to persuasion [4], the primary focus was on ensuring that the recipients find the content easy to understand, rather than determining whether they grasp the content accurately.

Participants filled in the persuasiveness and resistance scales which have been proven to be reliable and valid in Japanese people [33, 34]. The persuasiveness score was calculated as the average of 7 items scored on a 1-to-5 point Likert scale; higher scores indicate that the message is more acceptable and persuasive to the audience. The resistance score was calculated as the average of 6 items scored on a 1-to-5 point Likert scale; higher scores indicate that the audience feel greater reactance or apathy to the message. The overall score was calculated as the persuasiveness score minus the resistance score.

Participants were asked ‘When the poster is hanging on a wall at a station yard, what would you do?’ with 4 response options (not become aware/take no notice/throw a glance/stop to look) [34]. The responses were dichotomized into ‘pay attention’ (throw a glance/stop to look) and ‘pay no attention’ (not become aware/take no notice).

In addition to the questions about a given poster, participants were asked whether they were interested in their health (i.e. health awareness) and whether they had been encouraged to do as the poster suggested (i.e. previous contact). They also provided their demographic information (gender, age, marriage, education, occupation, and household income).

All statistical analyses were performed using the SAS ver. 9.4 (SAS Institute, Cary, NC, USA). Significant levels were set at p < 0.05. One-way analysis of variance was used to compare the mean scores among the 3 different design posters in the Survey1. Two-way analysis of variance was used to determine the effect of 2 factors (frame and tone) on the scores in the Survey2. The percentages of participants who would pay attention to the poster (i.e. success rate of attention attraction) were compared using Chi-square test.

The participants in the surveys were not involved in the design, conduct, or reporting of this study.