If you read the results report, you will see there was a control experiment.
The results can be read here: https://psywb.springeropen.com/articles/10.1186/s13612-016-0044-4
Here is the conclusion from the actual report:
Conclusions
We aimed to leverage the prevalence of smartphone photography along with theories of positive psychology to help college students become happier and reduce stress. To this end, we conducted a 4-week study with 41 participants to investigate the effects of taking daily photos using their smartphones in three conditions: the Selfie condition in which participants took a smiling selfie, the Personal condition in which participants took a photo of something that made themselves happy and the Other condition in which participants took and sent a photo of something to make another person happy. Quantitative and qualitative results show that participants in all three conditions became more positive after taking their assigned type of photo daily for 3 weeks. Some participants in the Selfie condition observed a more natural smile over time; participants in the Personal condition became more reflective and some participants reported that the photos led them to be more appreciative of the little things in their lives that made them happy. Participants in the Other condition became much less aroused (i.e., calmer) with photo-taking and some reported the increased intimacy and connection with strong ties as an important factor that can reduce anxiety, serve to pacify themselves and lead them to become more positive. Compared to photos posted on social media, participants felt more comfortable, conscious, and reflective when taking the photos. They also suggested future technology that could help them take and review photos of happy moments using mood-tracking sensors.
This paper provides empirical support on the feasibility of increasing users’ happiness by applying positive psychology to smartphone photography. It also contributes to the emerging field of positive computing by presenting reasons for how conducting exercises to promote happiness using mobile technology could help people enhance their mood. The findings can offer insights for designers to create systems that enhance emotional well-being.