The lawyers that address this topic in online articles seem to present the Internet-user with confusing information. For example, one article states that lack of a seat belt does not always diminish the chances for winning in a lawsuit. Another article indicates that a damage reward becomes lower, if a substantial number of damages could have been prevented by the wearing of a seat belt.
The legal question seems to rest on the number of damages, and not the extent to which a single damage might alter the life of the person most affected by that same damage. Moreover, no mention has been made online of the number of seatbelts that a given driver or passenger might be required to wear.

How do those facts relate to an accident injury claim?

You can file a claim, even if you were not wearing a seat belt. While that fact cannot be disputed, there is also no guarantee that your chosen lawyer will welcome the chance to pursue charges, if you have been neglectful. Evidence of negligence could weaken your case.
At times, the lawyer for the victim in an accident might even worry about ways that a defense lawyer could imply negligence on your part, while questioning you in a courtroom. Indeed, there has been at least one case in which a victim’s lawyer expressed concerns, because she had not been wearing two seat belts.

Why would a lawyer introduce the possible need to wear two seat belts?

As stated above, the victim of a car accident will not have a strong case, if he or she was neglectful and thus increased the chances for sustaining a costly injury. In a courtroom, where few people understand a victim’s pre-existing condition, it might appear risky for that same person to depend on a single seat belt. Indeed, that was fear that filled the mind of one personal injury lawyer, a man that was handling an accident injury case.
His client had a pre-existing condition. She could be sure of good health only when and if her ventricular shunt was functioning properly. That ventricular shunt drained fluid from her skull. Consequently, an impact to her head could affect its ability to function as it should.
He spoke with the client about the possible accusation that could be made by a defense lawyer: the accusation that the absence of a second seat belt had aggravated the extent of her injuries. After the accident, testing showed that her shunt was no longer working. She did win some money, but not as much as she had hoped. Should she have put that added seat belt in her car?
However, the fact is that just because she was not wearing a seatbelt does not mean that the defendant can be absolved of bad driving which led to the accident. As per the Ontario Court of Appeal, the victim’s damages can be reduced by 25% if contributory negligence is found and it could have been avoided, if the seatbelt was worn.