Hansen's interest in livestock species started early in life while staying with relatives in County Wexford Ireland He first did research while an undergraduate in the Dept. of Dairy Science at the University of Illinois under the tutelage of Charles E. Graves. His doctoral research focused on regulation of reproduction by photoperiod in cattle and mice. Postdoctoral work with Roberts and Bazer kindled a love for understanding the establishment and maintenance of pregnancy, which subsequently became a career-long research interest. Among the most notable achievements as a faculty member at Florida has been identifying embryokines that regulate development of the preimplantation embryo, demonstrating sex-dependent developmental programming during the preimplantation period, understanding how elevated temperature compromises reproduction, development of embryo transfer to increase pregnancy rate in heat-stressed cows, demonstration of the importance of the slick mutation in the prolactin receptor gene for increasing thermotolerance of cattle, and characterization of mechanisms for inhibition of uterine immune function by progesterone.