You don't say what your legal situation is and whether you already have a visitation schedule in place that the child's mother is ignoring. If you do and if she's not adhering to it, you can file a contempt complaint; you can request that she cover your expenses and court costs. That may be enough to motivate her. If you have an informal arrangement and the child's mother set 18 months as the dividing line and is now hesitating, then you need to go to court to get a better agreement.
I don't see why you should have to research the issue to prove why children should spend time with both parents. Short of a nursing infant or a child with some specific medical needs that make shifting back and forth a hazard, or short of a father with some sort of legal or criminal or behavior issue, there should be no discussion. You have to have a crib and safety precautions in place (gates, cabinet locks, that sort of thing), but I'm assuming you do.
It shouldn't be hard to argue (and hopefully you wouldn't have to document) that:
- children do better if they have loving relationships with both parents (which is why courts routinely give shared custody)
- children do better if they develop a self-confidence and can go to sleep in different places put to bed by different people: babysitters at either home, vacation/hotel cribs, grandparents' house, and so on.
- children do better if they are secure in the knowledge that multiple people love and care for them, so they are not insecure when the primary caregiver is absent (illness, in a hospital giving birth to a subsequent child, traveling for business, off on a fun weekend...the list is endless).
- children who cannot separate from Mom or Dad don't do well in other situations - babysitters, grandparents, preschool, kindergarten, Cub Scout camp-outs, sleepovers with friends. Waiting until a child is 2 or 4 or 7 or 12 to allow overnights just prolongs the learning process.
Your child needs you.
Good luck.