Psychologists who work on divorce cases are sometimes referred to as independent custody evaluators or advisors. Consider hiring one to assist you with making a convincing case as to why you should be awarded custody. Although the service can be costly, it can make the difference between the outcome you want, and a less attractive one that may be very hard to live with.
Choose a psychologist who testifies for both fathers and mothers. Otherwise, you run the risk of the Court viewing your expert as biased, and lacking credibility. You want someone with substantial experience in handling custody cases, who has a history of having been on the winning side of the battle at least 75% of the time. Also, the judge is likely to regard an expert who is board certified in pediatric psychiatry as having more authority.
Equally important is making sure that your expert has not received substantial income from the law firm representing your spouse. He or she may then be conflicted by a feeling of loyalty, or an interest in future employment by that firm, and not make as vigorous a case as otherwise. Have your lawyer investigate the person's background thoroughly.
The decision as to whom to hire is vital. If at any point you have doubts about the psychologist wholeheartedly recommending that you get the custody arrangement you're seeking, express them to your attorney and discuss hiring someone else. If you choose the wrong expert and have to switch, your spouse's attorney may subpoena the original one to testify against you. Nevertheless, it's better to take that risk than to continue with an expert who won't fully support you.
The earlier you finalize the choice of an expert the better.