I would have expected that she would have succumbed at an early age, not lasting into adulthood before she lost control to the others who would control her.
Alia's loss of control (to the Baron) is provoked by her taking ever-increasing overdoses of spice in order to try to see her visions; of Paul's death, her own death and the subsequent reign of terror by his son, more clearly.
He studied her a moment. Then: “You know the answer as well as I do. They had secure personalities by the time they came to Arrakis. The spice trance—well …” He shrugged. “They weren’t born into this world already possessed of their ancestors. Alia, though …”
“Why didn’t she believe the Bene Gesserit warnings?” Ghanima chewed her lower lip. “Alia had the same information to draw upon that we do.”
“They already were calling her Abomination,” Leto said. “Don’t you find it tempting to find out if you’re stronger than all of those …”
“We must delay and delay and delay every time she urges the trance upon us. Extreme caution with a spice overdose; that’s our best course.”
“An overdose would have to be pretty large,” Ghanima said.
“Our tolerance is probably high,” he agreed. “Look how much Alia requires. ”
“I pity her,” Ghanima said. “The lure of it must’ve been subtle and insidious, creeping up on her until …”
“She’s a victim, yes,” Leto said. “Abomination.”
- Children of Dune