According to the meta-analysis by Rind et.al. 1998, the use of force is the abuse-related variable with highest influence on the impact.
Other sources are in agreement with this result. For example, Browne & Finkelhor 1986 write: "Five studies, three of which had difficulty finding expected associations between trauma and many other variables, did find an association between trauma and the presence of force. With Finkelhor's (1979) student samples, use by force by an abuser explained more of a victim's negative reactions than any other variable and this finding held up in multivariate analysis. Frohmuth (1983), in a replication of the Finkelhor study, found similar results. In Russel's (in press) study 71% of the victims of force rated themselve as extremely or considerably traumatized, compared with 47% of the other victims."
Beitchman et al. 1992 write "Anxiety, fear, and suicidal ideas and behavior have also been associated with a history of childhood sexual abuse but force and threat of force may be a necessary concomitant. [] Longer duration is associated with greater impact, and the use of force or threat of force is associated with greater harm." Molanr et al 2001 found that among women, rape (vs molestation), knowing the perpetrator (vs strangers), and chronicity of CSA (vs isolated incidents) were associated with higher odds of some disorders.
The only result which points partially in the different direction is Mennen 1995: "Force predicted higher levels of distress on [depression and self-worth] measures when the perpetrator was not a father figure and lower levels of distress when the abuser was a father figure".
For the 11 incest cases of severe chronic mental illness considered by Bigras et al. 1998, the incest for occurred over a long period of time and was violent.