Unintended reasoning: understanding designer design activity towards user interaction experience and behaviour in mainstream product

Mainstream product design has long prioritised the alignment of human requirements with technological advancement through a human-centred design approach. However, a significant gap persists in addressing unintended human behaviour (UHBE) the unexpected ways users engage with products in their daily lives. In design research, this phenomenon is often examined from a limited perspective across various fields, leaving designers with few methodologies to comprehend the underlying causes, interpret human interaction, and integrate these insights meaningfully into the design process. A more profound understanding of UHBE is therefore essential, as it influences humanproduct interaction, communication, and the overall user experience, which in turn can shape product innovation and design outcomes. This study investigates UHBE as a central phenomenon in mainstream product design represented by a wash basin product in exploring both user perceptions and designer reflections through the 'consumerism' model, which is deemed to be closer to the user's understanding. Data was collected via a questionnaire survey to examine people's experiences and perceptions regarding the unintended use of products. Further analysis included a visual inventory of user interactions, photo-based research to assess designers' responses to behavioural attributes, and verbal protocol analysis to capture designers' reflective thinking during design activities. Additionally, design syntactic analysis was employed to interpret the structural aspects of design form and its role in establishing unintended use behavioural models. These approaches collectively provide a multi-layered understanding of how designers interpret, respond to, and integrate user behaviour into their creative practices. The findings of Visual Inventory Analysis (VIA) indicate that unintended behaviours closely relate to users' implicit needs for comfort, recognition, safety, and adaptation, while also reflecting designers' interpretations of user experience within the design process. The result of Photo-based research analysis (PBA) reflecting users and designers demonstrate different patterns of perception towards UHBE attributes. The diverse perceptions reflect the gap in understanding of the product, which highlight the importance of integrating both perspectives during the early stages of the design process to enhance usability, functional clarity, thereby supporting human-centered & sustainable product development grounded in real-world interaction. The study emphasises UHBE's ability to inform design thinking, stimulate reflective practice, and enhance methodological approaches in product development through Verbal Protocol Analysis (VPA) and the proposed Unintended Behavioural Design (UBD) Framework for product development strategy based on Design Syntactic Analysis (DSA) findings. It argues that recognising unintended use not only enriches designers' understanding of everyday interaction but also contributes to a more responsive and innovative mainstream product design. The researcher concludes that a hermeneutical and reflective design approach offers a valuable pathway for integrating user behaviour into the design process. By encouraging deeper engagement with UHBE, this study contributes to the expansion of the body of knowledge, has implications for design pedagogy, design thinking, refinement of designer criteria in everyday design practice, and cultivation of creativity in design education, ultimately supporting the development of more adaptive and meaningful product ideas in the development of a conceptual design.