According to the online resource The Free Dictionary, duress is an act that puts pressure on or coerces a person to undertake actions that he would not ordinarily choose to undertake. These acts include the use of threats, mental coercion, physical harm or illegal imprisonment or confinement. Family law agreements include divorce settlements, child custody, support and visitation, as well as pre- and post-nuptial agreements. Signing these agreements is supposed to be on a mutual basis. If you sign these documents as result of coercion, then this constitutes duress in family law.
A family law agreement is considered to be signed under duress if one of the parties does not disclose information. This may result in one party signing into an agreement he might not otherwise have signed. Such an agreement that was made without full disclosure may not be enforced by the court or can be revoked by the court afterwards. For example, a wife may file to re-open a divorce case or revoke a property agreement on the basis of duress if the husband did not disclose information about some assets.
Threat and Violence
The use of violence or threats to force a person to sign an agreement is considered duress in family law. If violence and threats are also used on the wife's or the husband's family, then this constitutes duress too. For example, a husband who agrees to a spousal support settlement after his wife had physically and verbally abused him can claim that the settlement was signed under duress of threat and violence.
Against Free Will
A family law agreement signed without one party's free will is considered to be signed under duress. This party must demonstrate that the actions of the other party prevented her from exercising free will. In this case, the court will assess the mental state of the person alleging duress as well as conditions under which the agreement was signed. If she was not in the proper mental state, then duress can be established. For example, a wife who is coerced to sign an agreement while she is in the hospital can claim that this was against her will.
No Chance for Negotiation
When a person signs an agreement because there is no reasonable chance of negotiating or compromising with the other party, then this may constitute duress. For example, a housewife may fear that her financially or socially well-connected husband may make life difficult for her if she takes a different position in an agreement. If she does not have a readily available alternative, she will sign the agreement but may still claim duress later. She has to prove to the court that she had no chance to negotiate or compromise with her husband.