B6D2F2/J mice were purchased from The Jackson Laboratory. Random mating pairs were established to produce an F3 hybrid cross, which was subsequently used as the founding population. Each F3 individual was genetically unique, possessing either C57BL/6J or DBA/2J alleles at each gene locus, and had experienced multiple recombinations of the progenitor strains’ chromosomes. One hundred eleven 55- to 65-day-old naïve B6D2F3 mice (53 male and 58 female) were tested on the hole-board as described for the inbred strain experiments. The 12 male and 12 female mice that showed the most head dipping behavior in the 10-min task were randomly divided into two replicate lines of six breeder pairs each to form the HEB-1 and HEB-2 lines, whereas the lowest-scoring 12 male and 12 female mice were paired to form the LEB-1 and LEB-2 lines. A mass selection procedure was used; mice were randomly assigned mates so long as the selected mate was not a sibling. This procedure was chosen to produce the fastest rate of response to selection at the expense of somewhat higher rates of inbreeding. After being selected
