This paper addresses the development of an adaptive cooperative agent in a domain that suffers from human error in the allocation of attention. The design is discussed of a component of this adaptive agent, called Human Attention-Based Task Allocator (HABTA), capable of managing agent and human attention. The HABTA-component reallocates the human's and agent's focus of attention to tasks or objects based on an estimation of the current human allocation of attention and by comparison of this estimation with certain normative rules. The main contribution of the present paper is the description of the combined approach of design and validation for the development of such components. Two complementary experiments of validation of HABTA are described. The first experiment validates the model of human attention that is incorporated in HABTA, comparing estimations of the model with those of humans. The second experiment validates the HABTA-component itself, measuring its effect in terms of human-agent team performance, trust, and reliance. Finally, some intermediary results of the first experiment are shown, using human data in the domain of naval warfare.