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彼はストレスに弱い傾向にある。 He tends to be weak against stress.

彼は他人をばかにする傾向がある。He is apt to ridicule others.

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Strictly speaking, the former one "ストレスに弱い傾向にある" is wrong. It should be "ストレスに弱い傾向がある". But in this case, when you make a daily conversation, it's not strange to use "傾向にある", so you don't have to be worried.

"傾向にある" is kind of "changing right now", for example

最近、その国での犯罪率は増加の傾向にある (Lately, the number of crimes in the country is increasing)

In this case, "傾向にある" is right because the number of crimes is "still" increasing. And you can NOT use "傾向がある". But of course, if this is a daily conversation, it is no problem to use "傾向がある" (although it sounds strange a little bit).

"傾向がある" is "not changing right now", for example

彼は他人をばかにする傾向がある。(He is apt to ridicule others.)

In this case, "He" is not changing to ridicule others right now, but he has been apt to ridicule since sometime and still he is.

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