Why do some parents practice tough love to their older children but spoil the younger one? What is the reason behind it?

It's common nature I guess, where the parents may have smothered the elder one with love and pampering in the younger days, but after a certain point that changed to tough love. As the child grows, parents realize that the child is now capable of understanding and taking on responsibilities, and hence the change in attitude. When a younger baby comes in the picture, the attitude again changes where the parents start showering too much love. Of course discipline should be practiced at all age, and spoiling with too much love and easy access is not the right way to go I feel.

I don't think the younger child gets spoiled. If the kids are close in age there is hardly any difference in treatment. Only where the elder child is 5 years older or more there will be some difference. ...for obvious reasons. But you cannot generalize this.

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Not in my case! I am equally strict with both my children. You cannot generalize this! Sometimes the younger one is spoilt as he/she is the youngest of the house.