How to compare fluctuating asymmetry of different traits

Comparing fluctuating asymmetry (FA) between different traits can be diffic ult because traits vary at different scales. FA is generally quantified eit her as the variance of the difference between left and right (sigma(L-R)(2) ) or the mean of the absolute value of this difference (mu(\ R-L \)). Corre ctions for scale differences are obtained by dividing by trait size mean. W e show that a third index, one minus the correlation coefficient between le ft and right (1 - r(L,R)), is equivalent to sigma(L-R)(2) standardized by t rait size variance. The indices are compared with Monte-Carlo simulations. All achieve the expected correction for scale differences. High type I erro r rates (false indication of differences) occur only for sigma(L-R)(2) and mu(\ R-L \) if trait sizes close to or below 0 occur. 1 - r(L,R) with a boo tstrap test has always low error rates. Recommendation of the index to be u sed should be based on whether standardization of FA by trait size mean or trait size variance is preferred. A survey of 36 traits in the Speckled Woo d Butterfly (Pararge aegeria) indicated that sigma(L-R)(2) is slightly high er correlated to trait size variance than to trait size mean. Thus 1 - r(L, R) seems to be the superior index and should be reported when FA of differe nt traits is compared.