I'm testing for gender differences an autoregressive cross-lagged model using a multiple group model (boys vs. girls). First I let the parameters free for boys and girls. Then I rerun the model with the parameter estimates equal between boys and girls. As a result, the chi-square of the model with free estimates has a higher chi-square (and of course fewer Df) than the one with the fixed parameters, resulting in a negative chi-square difference value.
Is a negative chi-square difference acceptable? And if yes, shall I report the value with the negative sign?
What did you mean by "the results are not meaningful"? Does it mean that the chi-square test is not valid? Or that the differences I've found between boys and girls in the free estimated model are not significant and are the results of the reduced sample sizes of model male and female? Or maybe something else?