Damaged parties are only liable for causally caused cartel damage. § Section 37c (1) KartG clarifies the causality requirement by clarifying that the infringer must compensate for the damage caused.
The infringer is not liable for other causes of damage caused by third parties. According to the Conditio sine qua non formula, conduct is causal for success if it cannot be ignored without losing its concrete form. A prohibited price agreement is, for example, causal for the damage to the downstream economic level if the price increase would not have occurred without the agreement of the cartel members. If the prices rise not because of agreements, but exclusively because of the passing on of the increased raw material prices, the agreement is not causal for the price increase. In this case the prices would have risen to the same extent if no agreement had been reached.
In practice, it is therefore necessary to analyse exactly what the concrete behaviour was causal for or which other influencing factors, which were caused by third parties independently of the cartel, influenced the concrete event to be assessed. Defendants can evade liability if they can prove that it was not the actions they were accused of by the plaintiff that caused the damage, but other causes.
Details can be found in the book Michael BRAND, Schadenersatz im Kartellrecht, starting on page 192.