Rutte and Taborsky (2007) tested for the existence of “generalized reciprocity” in the Norway rat, Rattus norvegicus. That is, they asked whether a rat’s willingness to help another individual would be affected by receiving help itself. Female rats were trained to pull a stick attached to a tray that produced food for their partners but not for themselves. Subsequently, each rat’s experience was manipulated in two treatments.

Under one treatment, the rat was helped by three unfamiliar rats (who pulled the appropriate stick). Under the other treatment, rats received no help from three unfamiliar rats (who did not pull the stick). Each rat was exposed to both treatments in random order. Afterward, each rat’s tendency to pull for an unfamiliar partner rat was measured. The number of pulls in a given period (in pulls/min.) by 19 female rats after both treatments is given in the following table.

a. Draw a graph to illustrate the data. What trend is evident?

b. What are the means of the two treatments, and what is the mean difference?

c. Test whether a difference was detectable between the help and no-help treatments.

d. Why is it important to apply the two treatments to the focal rats in random order?

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Rat 1010 1111 1212 2020 3030 3131 3232 3333 3434 4040 4141 4242 4343 5050 5151 5252 6060 6161 6262 After receiving help 0.430.43 0.860.86 0.570.57 0.860.86 0.290.29 1.141.14 0.570.57 0.860.86 1.431.43 0.860.86 0.570.57 0.860.86 0.000.00 1.001.00 0.860.86 0.860.86 1.861.86 0.860.86 0.860.86 After receiving no help 0.290.29 0.140.14 0.290.29 0.570.57 0.290.29 0.710.71 0.290.29 0.860.86 0.860.86 0.860.86 0.430.43 1.001.00 0.860.86 0.430.43 1.001.00 0.000.00 1.141.14 0.710.71 0.710.71