In recent years, smartphone devices are becoming progressively popular across a diverse range of users. However, user diversity creates challenges in smartphone application (app) development. The diversity of users is often ignored by designers and developers due to the absence of requirements. Owing to this, many smartphone users face usability issues. Despite that, no dedicated platform found that guide smartphone app designers and developers regarding human universality. The aim of this research is to explore the requirements of diverse users in smartphone apps and provide usability guidelines. The objectives of this research are achieved by following two scientific approaches. The human diversity requirements are located by conducting usability tests that investigated the requirements in the form of usability issues. The systematic literature review (SLR) process is followed in order to resolve the discovered usability issues. Both approaches resulted in a list of usability issues and guidelines. The usability tests returned 27 problems while the SLR came with a comprehensive set of universal usability guidelines that were grouped into eleven categories. The study concluded with some major outcomes. The results show evidence of critical usability problems that must be addressed during the design and development of smartphone apps. Moreover, the study also revealed that people with disabilities were three times severely affected by usability problems in such apps than people of different ages and their needs must be considered a top priority in the development of smartphone apps.
