BEHAVIORAL DIFFERENCES 



289 



Another difficulty is encountered when several behavioral patterns are used to 

 identify a particular trait. For example, sniffing of the female genitals is part of the 

 male sexual behavior of guinea pigs. 510 This behavioral pattern, however, actually 

 appears more frequently in low-sex-drive males than high-drive males. The latter 

 proceed directly to more active courtship and copulation. As indicated above, the 

 measurement of behavior tends to result in a quantitative analysis of the effects of 

 genetics. However, behavior may also be analyzed in terms of the presence or absence of 

 specific patterns, and the possibility of unitary characters is not excluded. 



Choice of conditions for testing. — -Conditions of stimulation are critical for the extension 

 of group differences. This is expressed diagrammatically in figure 39, which shows 



Fig. 39. Hypothetical relationship between stimulus and intensity of response in 



two populations, A and B. 



I 2 3 4 5 6 7 8 9 10 II 12 

 STIMULUS INTENSITY 



an hypothetical relationship between stimulus and response in two populations, A 

 and B. Note that below a stimulus threshold, 3 for B and 4 for A, no response is 

 elicited in either population. On this measurement, therefore, B would be considered 

 the more responsive. As intensity of stimulus increases up to 6, the differences 

 between populations also increase. Above 6, they converge until they are equal at a 

 stimulus intensity of 8. At still higher levels of stimulation, responses are on a plateau 

 and a physiologic limit has been reached. 



Obviously, measurements of strain differences which follow this model are depen- 

 dent upon the particular level of stimulus in the situation for testing. Measurements of 

 threshold stimuli have not been widely used in behavioral genetics, although any 

 test which involves the presence or absence of a behavioral pattern probably involves 

 such a threshold. Measures taken at a physiologic limit have the advantage that 



