Oliver is a supervisor at a construction company. His job is to oversee the construction workers' work in terms of quality and productivity. He often feels that despite all efforts, his instructions are not being taken seriously or that he is being ignored by the workers. Upon seeking feedback, Oliver is surprised to learn that he comes across as too passive and his team sees him as a non-assertive person who cannot exert control. In the context of understanding individual behavior, Oliver's lack of assertiveness is his _____.