Negative constant for income How can I interpret the negative constant of -40348.86 on the following model?
 =  +  +  + 33 + 
Income=  + Age + Height + 3Sex + 
Income= -40348,806 + 1891,185 age +  423,238 height +12215,847 sex
Sex has been coded as male=1 and female=0

The intercept of -40348,806 is the prediction if all predictors have a value of zero.
Thus, a woman (sex = 0) of age 0 and height 0 would be predicted to have a negative income of -40348,806.
This is obviously completely irrelevant, since you presumably have no newborn girls of zero height in your sample. It illustrates that models often are nonsensical when extrapolating outside observed predictor settings.
