>>260143Probable ESL issue.
"Stronger" as in more powerful, a higher number of capacities. People nowadays can travel long distances, live longer, know and do more things, they have more abilities.
"less agressive" : less interpersonal violence, less risks, more protection in our current world. As in: no wolves, thieves, or plague outside your door whenever you need to find food.
Before, you needed a big ass family to survive a disease taking your kids or your harvest. Now you can be alone and more or less deal with whatever the world is sending your way.
So we don't need anymore to live as groups (family) or even as couples. Hence the reduced number of durable relationships; the social incentive is much less important.