What is the biblical basis for the belief in generational curses? I've been watching testimonies of people who claim to have been involved in deliverances and spiritual warfare, and the concept of generational curse is brought up quite often. It's even taught as a doctrine in certain churches (e.g. see this Hungry Generation's sermon). In simple words, a generational curse basically means that some ancestor messed up (e.g. by dabbling into witchcraft, by practicing the occult, by doing some terrible sin, etc.) and, thus, gave to the dark side a legal right to influence them and their descendants in some manner, until the right gets eventually revoked (if ever) by someone down the lineage. What is the biblical basis for this belief?
I’m not prepared to defend the beliefs detailed in that link, nor do I think that the biblical statements of God in Exodus 20:4-6 apply to the idea you ask about. God depicts idol worshippers, and those who worship sun, moon, or stars, as having God’s curse brought down, up to the third and fourth generation “of them that hate me”. In contrast, God says he will show mercy to “thousands of them that love me, and keep my commandments.” The Old Testament testifies to the truth of God’s warning with regard to later generations of his own people who turned to idolatry.
However, the Church Jesus establishes, which “the gates of Hades will not overcome” (Matthew 16:18) are commanded, “Little children, keep yourselves from idols.” (1 John 5:21)  Further, all legal rights pertaining to the Church Jesus causes to grow belong to the risen Christ, who purchased her with his own blood (Acts 20:28). This means that where there are Spirit-born members of the congregation of Christ, keeping themselves free from idolatry (in all its forms), nobody and nothing can lay “a generational curse” upon them.
Yet there remains a real problem with many who associate with the Christian congregation who may have a family history of idolatry. This is where different biblical principles apply. For a start, there is the one of sowing and reaping. “Whatever a man sows, that he will also reap” (Galatians 6:7). Many times, children suffer the bad results from what their parents have ‘sown’. But the matter you ask about goes deeper and can be illustrated by the experiences of two Christian friends of mine who spend several months each year in India, one of the most idolatrous nations on earth.
They maintain that the principles in Hebrews 12:14-15 &amp; Ephesians 4:31, which warn Christians against allowing bitter roots to develop, is what can bring judgement (even to the third or fourth generation). Here is a true account from one of their letters to me (therefore, no link). They spoke with a young Hindu woman who was interested in learning about Christianity, and now I quote:

&quot;Her father had died from drowning in an alcohol-related incident when
he was only 28 and now her husband is drinking alcohol heavily (to the
point of collapse) every day. We started talking about how sins
sometimes ‘run’ in families and that in the exact way that someone
dishonours their parent(s), things do not go well with them. We then
asked her if she had ever judged her father in any way. It turned out
that she had hated him in her heart, thought he was a bad person
because of his drinking and beating of her mother, and she vowed that
she would never marry anyone like that’.
&quot;We explained to her the teaching we had received from Elijah House on
'bitter-root judgments'. She was willing to repent of dishonouring her
father, for judging him and also for judging her husband in the same
way; then she forgave them both.
&quot;As she prayed she saw a picture of a wooden cross in front of her;
she was filled with joy and knew in her heart that she was forgiven.
&quot;We were able to pray with the woman's mother and found that she too
had judged her father and subsequently her husband to be bad because
of their alcoholism and violence. Her father had also taken a second
'wife'... Like her daughter, she had reaped what she had sown.
&quot;The woman's aunt had also judged her father as a bad man, partly for
his drinking and beatings but also for taking a second 'wife'. Her
mother, aunts and uncles had suffered terribly because their father
had two wives, two homes and two sets of children. And as we prayed
the aunt also saw that she had reaped what she had sown. With eyes
fully open and having experienced a difficult life from childhood, she
then went on to become the second 'wife' of a man who became violent
and alcoholic! She too repented and forgave. The marvellous good news
is that Jesus specialises in transforming messed-up lives.
&quot;The following day we were all amazed when the husband of the young
woman asked us to pray with him! No-one was more surprised than he
was! He prayed a prayer of repentance for his drinking and said he
felt happy afterwards... since that day he has not taken alcohol (even
over New Year). His wife's bitter-root judgements that were defiling
him had been broken and he was no longer feeling the compulsion to
drink.”

Now, that is the experience of my Christian friends. I know personally that when I once allowed bitterness and resentment against my good husband to develop over several years – despite the Lord warning me through scripture and the testimonies of other ladies – I ended up suffering the Lord’s chastisement.  That brought me to repentance, and healing (spiritual, physical and marital) resulted. But I do believe that had I not eventually obeyed God and dug out that bitter root, our children would have gone on to suffer, perhaps their children too. It doesn’t bear thinking about. That is why I flag up this principle of “bitter roots judgment” as a main one that applies to Christians. But I stoutly deny that any “dark forces” can gain a legal foothold into the lives and families of the children of God, born again of the Spirit.