Are happy couples better parents?

Perhaps best answered with another question, keithold? = Do properly inflated tires perform better?
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;-)

Its possible yes :)

they could, coz when you're happy you are free from stress and when you are free from stress the more you could focus and the more you could focus the more you could give your best to the children or to the people that you love

Academic comparisons of life outcomes and/or well-being between people who grew up with happy parents and people who grew up with unhappy parents suggest very strongly that it is, as one would expect, better to have happy parents. In other words, if you define "good parenting" as bringing up a child to become a happy, secure, successful adult, then the answer is a big YES. Interestingly, couples who separate tend to remain mediocre parents even after the relationship is over, which suggests that the problem might not be with the couple, but with the dispositions of the individuals in the couple.

Definitely yes!A child can't be happy if the parents are not!

Happy, stable marriage is always the best circumstance for a child.

can be yes or no, depends on the children. Parents needs to have patience, understanding and wisdom from God.

No matter how hard a parent tries to not show their unhappiness to children, they will always feel it in one shape, way or form. So even if they can't pin point what is bothering them, the will feel insecure about there family and it will translate to other insecurities down the road.