"They find evidence that large investors tend to trade more in periods ahead of important announcements, say, which is hard to explain unless they have access t

"They find evidence that large investors tend to trade more in periods ahead of important announcements, say, which is hard to explain unless they have access to unusually good information." Typically for regular macro-level financial announcements like GDP growth, employment numbers/non-farm payrolls, rates announcements, etc, the announcements happen on well-defined, publicly known intervals. Similarly for company specific things like earnings announcements. It is expected that there will be lots of (non-insider) trading before these announcements - active investors may just want to reduce risk before a large move in an unknown direction. Of course, I don't doubt that there is a large amount of insider trading, but its worth considering other causes of an observed phenomenon rather than just jumping to conclusions.