As preschoolers, the children born to the women who were diagnosed with both PTSD and depression following the September 11, 2001, attacks on the World Trade Center showed evidence of higher reactivity to stimuli and more aggressive behavior than did children of terrorism-exposed mothers who did not develop these disorders. What is the likely source

The ability of cortisol and other stress hormones to cross the placenta

Explanation:

Pregnant women who experienced traumatic events of September 11, 2001 can transmit these to the unborn child. Women who developed PTSD due to the attack were found to have lower cortisol levels in their saliva compared to other pregnant women who experienced the attack but didn't develop PTSD.

Children born to women with PTSD were also found to have lower levels of cortisol than the others. These children born to traumatic mother's showed evidence of higher reactivity to stimuli and aggressive behavior.

According to Research, the probability of this happening is due to epigenetics mechanism- this is the study of inherited changes in gene activity that is not caused by changes in the DNA sequence.