Over-Requirement, manifested when a product or a service is specified beyond the actual needs of the customer or the market, is considered as a major risk in software development projects. This research empirically investigates whether Over-Requirement is partially due to emotional involvement of developers with software features they develop. Due to behavioural effects termed Endowment effect, I-Designed-it-Myself effect and IKEA effect, such involvement has been demonstrated when people come to overvalue physical items that they possess, self-design, or self-create. We conducted an experiment to explore these three behavioural effects in the context of a software development project. The 86 participants were randomly assigned to eight different experiment groups, according to whether they were responsible for (or not), specified (or not) and/or constructed (or not) a nice-to-have Over-Required software feature. The study's preliminary findings show that following these manipulations participants over-valued features that they were assigned to be responsible for, to specify or to construct, confirming our proposition that behavioural effects impact software development processes and influence Over-Requirement. The study is of relevance to theory about behavioural effects in software development and to practice via insights to Over-Requirement risk.