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Why including variable square in the regression change the sign of main variable

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Dear Stata users,

I am estimating a simultaneous equation model of two equations using _cmp_ command.

Code:
cmp (rinlbrf = rshlt_#  age rabplace  i.seperated##i.kid18_  i.married##i.kid18_ lesshighschool college collegemore whitecollar2_ bluecollar wealth midwest northeast west hispanic black rsmoke rdrink|| hhidpn:) (rshlt_ =rinlbrf#  age rabplace married seperated  hispanic black midwest northeast west lesshighschool college collegemore whitecollar2_ bluecollar wealth rsmoke rdrink rconde || hhidpn:) if (sex==1 & age<=65) ,ind($cmp_probit $cmp_oprobit) nolr
where
rshlt is health categorical variable, and rinlbrf stands for participation in the labor market

The result shows that the effect of age on both health and labor participation is negative as expected. However, when "age2" variable [age2=age*age] is included in the regression, the age effect is positive which doesn't make sense. why?

Thanks,
Maryam


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