Infants raised in orphanages lose height and head circumference the longer they remain in institutional care. When they join families early enough, they often catch up. Parents can shape babies' bodies and brains. Those kids have better learning and problem-solving abilities at 3 and 6 years of age than similar kids whose parents received no help. Parents who were abused themselves are more likely to abuse their own children if they don't get help.
Parents can pass their pain on to their children.
Parents can be negatively affected by their child's circumstances and find it even harder to parent. The growth of our kids is so plastic and wondrous that it is impossible to nail down cause or effect with certainty. The evidence is overwhelming that early experience with caregivers is particularly significant. Yes, that one fact is enough to give most parents the willies. But it can also reassure us that the way we love our children matters a lot.